Method and device for encoding/decoding image using color space conversion, and method for transmitting bitstream

ABSTRACT

Provided are an image encoding/decoding method and device. An image decoding method performed by an image decoding device according to the present disclosure includes the steps of: determining a quantization parameter of the current block on the basis of whether color space conversion is applied to a residual sample of the current block; determining a transform coefficient of the current block on the basis of the quantization parameter; determining the residual sample of the current block by using the transform coefficient; and resetting the value of the residual sample on the basis of whether the color space conversion is applied.

TECHNICAL FIELD

The present disclosure relates to an image encoding/decoding method anddevice. More particularly, the present disclosure relates to an imageencoding/decoding method and device using color space conversion(or,transformation), and a method for transmitting a bitstream generated bythe image encoding method/device of the present disclosure.

BACKGROUND ART

Recently, demand for high-resolution and high-quality images such ashigh definition (HD) images and ultra high definition (UHD) images isincreasing in various fields. As resolution and quality of image dataare improved, the amount of transmitted information or bits relativelyincreases as compared to existing image data. An increase in the amountof transmitted information or bits causes an increase in transmissioncost and storage cost.

Accordingly, there is a need for high-efficient image compressiontechnology for effectively transmitting, storing and reproducinginformation on high-resolution and high-quality images.

DISCLOSURE Technical Problem

The present disclosure is directed to providing an imageencoding/decoding method and device with improved encoding/decodingefficiency.

In addition, the present disclosure is directed to providing an imageencoding/decoding method and device for improving encoding/decodingefficiency by performing selective color space conversion.

In addition, the present disclosure is directed to providing a methodfor transmitting a bitstream generated by an image encoding method ordevice according to the present disclosure.

In addition, the present disclosure is directed to providing a recordingmedium storing therein a bitstream generated by an image encoding methodor device according to the present disclosure.

In addition, the present disclosure is directed to providing a recordingmedium storing therein a bitstream that is received, decoded, and usedto reconstruct an image by an image decoding device according to thepresent disclosure.

It will be appreciated by persons skilled in the art that technicalobjectives to be achieved in the present disclosure are not limited tothe above-mentioned technical objectives and other technical objectiveswhich are not described herein will be clearly understood from thefollowing description.

Technical Solution

According to an aspect of the present disclosure, there is provided animage decoding method performed by an image decoding device, the methodincluding: determining a quantization parameter of a current block onthe basis of whether color space conversion is applied to a residualsample of the current block; determining a trasnform coefficient of thecurrent block on the basis of the quantization parameter; determiningthe residual sample of the current block by using the trasnformcoefficient; and resetting a value of the residual sample on the basisof whether the color space conversion is applied. Herein, thedetermining of the quantization parameter may be performed by performingclipping on the quantization parameter such that a value of thequantization parameter has a value less than or equal to a predeterminedupper limit value and greater than or equal to a predetermined lowerlimit value.

In addition, according to another aspect of the present disclosure,there is provided an image decoding device including a memory and atleast one processor, the at least one processor is configured todetermine a quantization parameter of a current block on the basis ofwhether color space conversion is applied to a residual sample of thecurrent block, determine a transform coefficient of the current block onthe basis of the quantization parameter, determine the residual sampleof the current block by using the transform coefficient, and reset avalue of the residual sample on the basis of whether the color spaceconversion is applied. Herein, the processor may be configured toperform clipping on the quantization parameter such that a value of thequantization parameter has a value less than or equal to a predeterminedupper limit value and greater than or equal to a predetermined lowerlimit value.

In addition, according to another aspect of the present disclosure,there is provided an image encoding method performed by an imageencoding device, the method including: resetting a residual sample onthe basis of whether color space conversion is applied; determining atransform coefficient by using the reset residual sample; determining aquantization parameter on the basis of whether the color spaceconversion is applied; and encoding the transform coefficient on thebasis of the quantization parameter. Herein, the determining of thequantization parameter may be performed by performing clipping on thequantization parameter such that a value of the quantization parameterhas a value less than or equal to a predetermined upper limit value andgreater than or equal to a predetermined lower limit value.

In addition, according to another aspect of the present disclosure,there is provided a transmission method for transmitting a bitstreamgenerated by an image encoding device or an image encoding method of thepresent disclosure.

In addition, according to another aspect of the present disclosure,there is provided a computer-readable recording medium storing therein abitstream generated by an image encoding method or an image encodingdevice of the present disclosure.

The features briefly summarized above with respect to the presentdisclosure are merely exemplary aspects of the detailed descriptionbelow of the present disclosure, and do not limit the scope of thepresent disclosure.

Advantageous Effects

According to the present disclosure, it is possible to provide an imageencoding/decoding method and device with improved encoding/decodingefficiency.

In addition, according to the present disclosure, it is possible toprovide an image encoding/decoding method and device capable ofimproving encoding/decoding efficiency by performing selective colorspace conversion.

In addition, according to the present disclosure, it is possible toprovide a method for transmitting a bitstream generated by an imageencoding method or device according to the present disclosure.

In addition, according to the present disclosure, it is possible toprovide a recording medium storing therein a bitstream generated by animage encoding method or device according to the present disclosure.

In addition, according to the present disclosure, it is possible toprovide a recording medium storing therein a bitstream that is received,decoded, and used to reconstruct an image by an image decoding deviceaccording to the present disclosure.

It will be appreciated by persons skilled in the art that the effectsthat can be achieved through the present disclosure are not limited towhat has been particularly described hereinabove and other advantages ofthe present disclosure will be more clearly understood from thefollowing description.

DESCRIPTION OF DRAWINGS

FIG. 1 is a view schematically showing a video coding system, to whichan embodiment of the present disclosure is applicable.

FIG. 2 is a view schematically showing an image encoding device, towhich an embodiment of the present disclosure is applicable.

FIG. 3 is a view schematically showing an image decoding device, towhich an embodiment of the present disclosure is applicable.

FIG. 4 is a view showing a partitioning structure of an image accordingto an embodiment.

FIG. 5 is a view showing an embodiment of a splitting type of a blockaccording to a multi-type tree structure.

FIG. 6 is a view showing a signaling mechanism of block splittinginformation in a structure of a quadtree with nested multi-type treeaccording to the present disclosure.

FIG. 7 is a view showing an embodiment in which a CTU is partitionedinto multiple CUs.

FIG. 8 is a view showing a block diagram of CABAC according to anembodiment for encoding one syntax element.

FIGS. 9 to 12 are views showing entropy encoding and decoding accordingto an embodiment.

FIGS. 13 and 14 are views showing examples of picture decoding andencoding procedures according to an embodiment.

FIG. 15 is a view showing a hierarchical structure for a coded imageaccording to an embodiment.

FIG. 16 is a view showing neighboring reference samples according to anembodiment.

FIGS. 17 and 18 are views showing intra prediction according to anembodiment.

FIG. 19 is a view showing an embodiment of a decoding process to whichACT is applied.

FIG. 20 is a view showing an embodiment of a sequence parameter setsyntax table in which a syntax element related to ACT is signaled.

FIGS. 21 to 27 are views successively showing an embodiment of a syntaxtable of an encoding basis in which a syntax element related to ACT issignaled.

FIG. 28 is a view showing coding tree syntax according to an embodiment.

FIG. 29 is a view showing a method of encoding a residual sample ofBDPCM according to an embodiment.

FIG. 30 is a view showing modified quantized residual blocks generatedby performing BDPCM according to an embodiment.

FIG. 31 is a flowchart showing a procedure for encoding a current blockby applying BDPCM to an image encoding device according to anembodiment.

FIG. 32 is a flowchart showing a procedure for reconstructing a currentblock by applying BDPCM in an image decoding device according to anembodiment.

FIGS. 33 to 35 are views schematically showing syntax for signalinginformation on BDPCM.

FIGS. 36 to 51 are views showing a syntax table for signaling an ACTsyntax element according to each individual embodiment of the presentdisclosure.

FIG. 52 is a view showing an image decoding method according to anembodiment.

FIG. 53 is a view showing an image encoding method according to anembodiment.

FIG. 54 is a view showing a content streaming system, to which anembodiment of the present disclosure is applicable.

MODE FOR INVENTION

Hereinafter, the embodiments of the present disclosure will be describedin detail with reference to the accompanying drawings so as to be easilyimplemented by those skilled in the art. However, the present disclosuremay be implemented in various different forms, and is not limited to theembodiments described herein.

In describing the present disclosure, if it is determined that thedetailed description of a related known function or construction rendersthe scope of the present disclosure unnecessarily ambiguous, thedetailed description thereof will be omitted. In the drawings, parts notrelated to the description of the present disclosure are omitted, andsimilar reference numerals are attached to similar parts.

In the present disclosure, when a component is “connected”, “coupled” or“linked” to another component, it may include not only a directconnection relationship but also an indirect connection relationship inwhich an intervening component is present. In addition, when a component“includes” or “has” other components, it means that other components maybe further included, rather than excluding other components unlessotherwise stated.

In the present disclosure, the terms first, second, etc. may be usedonly for the purpose of distinguishing one component from othercomponents, and do not limit the order or importance of the componentsunless otherwise stated. Accordingly, within the scope of the presentdisclosure, a first component in one embodiment may be referred to as asecond component in another embodiment, and similarly, a secondcomponent in one embodiment may be referred to as a first component inanother embodiment.

In the present disclosure, components that are distinguished from eachother are intended to clearly describe each feature, and do not meanthat the components are necessarily separated. That is, a plurality ofcomponents may be integrated and implemented in one hardware or softwareunit, or one component may be distributed and implemented in a pluralityof hardware or software units. Therefore, even if not stated otherwise,such embodiments in which the components are integrated or the componentis distributed are also included in the scope of the present disclosure.

In the present disclosure, the components described in variousembodiments do not necessarily mean essential components, and somecomponents may be optional components. Accordingly, an embodimentconsisting of a subset of components described in an embodiment is alsoincluded in the scope of the present disclosure. In addition,embodiments including other components in addition to componentsdescribed in the various embodiments are included in the scope of thepresent disclosure.

The present disclosure relates to encoding and decoding of an image, andterms used in the present disclosure may have a general meaning commonlyused in the technical field, to which the present disclosure belongs,unless newly defined in the present disclosure.

In the present disclosure, a “video” may mean a set of images in aseries according to the passage of time. A “picture” generally means thebasis representing one image in a particular time period, and aslice/tile is an encoding basis constituting a part of a picture inencoding. One picture may be composed of one or more slices/tiles. Inaddition, a slice/tile may include one or more coding tree units (CTUs).One picture may be composed of one or more slices/tiles. One picture maybe composed of one or more tile groups. One tile group may include oneor more tiles. A brick may refer to a quadrangular area of CTU rowswithin a tile in a picture. One tile may include one or more bricks. Abrick may refer to a quadrangular area of CTU rows within a tile. Onetile may be partitioned into a plurality of bricks, and each brick mayinclude one or more CTU rows belonging to a tile. A tile that is notpartitioned into a plurality of bricks may also be treated as a brick.

In the present disclosure, a “pixel” or a “pel” may mean a smallest unitconstituting one picture (or image). In addition, “sample” may be usedas a term corresponding to a pixel. A sample may generally represent apixel or a value of a pixel, and may represent only a pixel/pixel valueof a luma component or only a pixel/pixel value of a chroma component.

In the present disclosure, a “unit” may represent a basic unit of imageprocessing. The unit may include at least one of a specific region ofthe picture and information related to the region. One unit may includeone luma block and two chroma (e.g., Cb, Cr) blocks. The unit may beused interchangeably with terms such as “sample array”, “block” or“area” in some cases. In a general case, an M×N block may includesamples (or sample arrays) or a set (or array) of transform coefficientsof M columns and N rows.

In the present disclosure, “current block” may mean one of “currentcoding block”, “current coding unit”, “coding target block”, “decodingtarget block” or “processing target block”. When prediction isperformed, “current block” may mean “current prediction block” or“prediction target block”. When transform (inversetransform)/quantization (dequantization) is performed, “current block”may mean “current transform block” or “transform target block”. Whenfiltering is performed, “current block” may mean “filtering targetblock”.

In addition, in the present disclosure, a “current block” may mean “aluma block of a current block” unless explicitly stated as a chromablock. The “chroma block of the current block” may be expressed byincluding an explicit description of a chroma block, such as “chromablock” or “current chroma block”.

In the present disclosure, the term “/” and “,” should be interpreted toindicate “and/or”. For instance, the expression “A/B” and “A, B” maymean “A and/or B.” Further, “A/B/C” and “A, B, C” may mean “at least oneof A, B, and/or C.”

In the present disclosure, the term “or” should be interpreted toindicate “and/or.” For instance, the expression “A or B” may comprise 1)only “A”, 2) only “B”, and/or 3) both “A and B”. In other words, in thepresent disclosure, the term “or” should be interpreted to indicate“additionally or alternatively.”

Overview of Video Coding System

FIG. 1 is a view showing a video coding system according to the presentdisclosure.

The video coding system according to an embodiment may include a sourcedevice 10 and a receiving device 20. The source device 10 may deliverencoded video and/or image information or data to the receiving device20 in the form of a file or streaming via a digital storage medium ornetwork.

The source device 10 according to an embodiment may include a videosource generator 11, an encoding apparatus 12 and a transmitter 13. Thereceiving device 20 according to an embodiment may include a receiver21, a decoding apparatus 12 and a renderer 23. The encoding apparatus 12may be called a video/image encoding apparatus, and the decodingapparatus 12 may be called a video/image decoding apparatus. Thetransmitter 13 may be included in the encoding apparatus 12. Thereceiver 21 may be included in the decoding apparatus 12. The renderer23 may include a display and the display may be configured as a separatedevice or an external component.

The video source generator 11 may acquire a video/image through aprocess of capturing, synthesizing or generating the video/image. Thevideo source generator 11 may include a video/image capture deviceand/or a video/image generating device. The video/image capture devicemay include, for example, one or more cameras, video/image archivesincluding previously captured video/images, and the like. Thevideo/image generating device may include, for example, computers,tablets and smartphones, and may (electronically) generate video/images.For example, a virtual video/image may be generated through a computeror the like. In this case, the video/image capturing process may bereplaced by a process of generating related data.

The encoding apparatus 12 may encode an input video/image. The encodingapparatus 12 may perform a series of procedures such as prediction,transform, and quantization for compression and coding efficiency. Theencoding apparatus 12 may output encoded data (encoded video/imageinformation) in the form of a bitstream.

The transmitter 13 may transmit the encoded video/image information ordata output in the form of a bitstream to the receiver 21 of thereceiving device 20 through a digital storage medium or a network in theform of a file or streaming The digital storage medium may includevarious storage mediums such as USB, SD, CD, DVD, Blu-ray, HDD, SSD, andthe like. The transmitter 13 may include an element for generating amedia file through a predetermined file format and may include anelement for transmission through a broadcast/communication network. Thereceiver 21 may extract/receive the bitstream from the storage medium ornetwork and transmit the bitstream to the decoding apparatus 12.

The decoding apparatus 12 may decode the video/image by performing aseries of procedures such as dequantization, inverse transform, andprediction corresponding to the operation of the encoding apparatus 12.

The renderer 23 may render the decoded video/image. The renderedvideo/image may be displayed through the display.

Overview of Image Encoding Apparatus

FIG. 2 is a view schematically showing an image encoding apparatus, towhich an embodiment of the present disclosure is applicable.

As shown in FIG. 2, the image encoding apparatus 100 may include animage partitioner 110, a subtractor 115, a transformer 120, a quantizer130, a dequantizer 140, an inverse transformer 150, an adder 155, afilter 160, a memory 170, an inter prediction unit 180, an intraprediction unit 185 and an entropy encoder 190. The inter predictionunit 180 and the intra prediction unit 185 may be collectively referredto as a “prediction unit”. The transformer 120, the quantizer 130, thedequantizer 140 and the inverse transformer 150 may be included in aresidual processor. The residual processor may further include thesubtractor 115.

All or at least some of the plurality of components configuring theimage encoding apparatus 100 may be configured by one hardware component(e.g., an encoder or a processor) in some embodiments. In addition, thememory 170 may include a decoded picture buffer (DPB) and may beconfigured by a digital storage medium.

The image partitioner 110 may partition an input image (or a picture ora frame) input to the image encoding apparatus 100 into one or moreprocessing units. For example, the processing unit may be called acoding unit (CU). The coding unit may be acquired by recursivelypartitioning a coding tree unit (CTU) or a largest coding unit (LCU)according to a quad-tree binary-tree ternary-tree (QT/BT/TT) structure.For example, one coding unit may be partitioned into a plurality ofcoding units of a deeper depth based on a quad tree structure, a binarytree structure, and/or a ternary structure. For partitioning of thecoding unit, a quad tree structure may be applied first and the binarytree structure and/or ternary structure may be applied later. The codingprocedure according to the present disclosure may be performed based onthe final coding unit that is no longer partitioned. The largest codingunit may be used as the final coding unit or the coding unit of deeperdepth acquired by partitioning the largest coding unit may be used asthe final coding unit. Here, the coding procedure may include aprocedure of prediction, transform, and reconstruction, which will bedescribed later. As another example, the processing unit of the codingprocedure may be a prediction unit (PU) or a transform unit (TU). Theprediction unit and the transform unit may be split or partitioned fromthe final coding unit. The prediction unit may be a unit of sampleprediction, and the transform unit may be a unit for deriving atransform coefficient and/or a unit for deriving a residual signal fromthe transform coefficient.

The prediction unit (the inter prediction unit 180 or the intraprediction unit 185) may perform prediction on a block to be processed(current block) and generate a predicted block including predictionsamples for the current block. The prediction unit may determine whetherintra prediction or inter prediction is applied on a current block or CUbasis. The prediction unit may generate various information related toprediction of the current block and transmit the generated informationto the entropy encoder 190. The information on the prediction may beencoded in the entropy encoder 190 and output in the form of abitstream.

The intra prediction unit 185 may predict the current block by referringto the samples in the current picture. The referred samples may belocated in the neighborhood of the current block or may be located apartaccording to the intra prediction mode and/or the intra predictiontechnique. The intra prediction modes may include a plurality ofnon-directional modes and a plurality of directional modes. Thenon-directional mode may include, for example, a DC mode and a planarmode. The directional mode may include, for example, 33 directionalprediction modes or 65 directional prediction modes according to thedegree of detail of the prediction direction. However, this is merely anexample, more or less directional prediction modes may be used dependingon a setting. The intra prediction unit 185 may determine the predictionmode applied to the current block by using a prediction mode applied toa neighboring block.

The inter prediction unit 180 may derive a predicted block for thecurrent block based on a reference block (reference sample array)specified by a motion vector on a reference picture. In this case, inorder to reduce the amount of motion information transmitted in theinter prediction mode, the motion information may be predicted in unitsof blocks, subblocks, or samples based on correlation of motioninformation between the neighboring block and the current block. Themotion information may include a motion vector and a reference pictureindex. The motion information may further include inter predictiondirection (L0 prediction, L1 prediction, Bi prediction, etc.)information. In the case of inter prediction, the neighboring block mayinclude a spatial neighboring block present in the current picture and atemporal neighboring block present in the reference picture. Thereference picture including the reference block and the referencepicture including the temporal neighboring block may be the same ordifferent. The temporal neighboring block may be called a collocatedreference block, a co-located CU (colCU), and the like. The referencepicture including the temporal neighboring block may be called acollocated picture (colPic). For example, the inter prediction unit 180may configure a motion information candidate list based on neighboringblocks and generate information indicating which candidate is used toderive a motion vector and/or a reference picture index of the currentblock. Inter prediction may be performed based on various predictionmodes. For example, in the case of a skip mode and a merge mode, theinter prediction unit 180 may use motion information of the neighboringblock as motion information of the current block. In the case of theskip mode, unlike the merge mode, the residual signal may not betransmitted. In the case of the motion vector prediction (MVP) mode, themotion vector of the neighboring block may be used as a motion vectorpredictor, and the motion vector of the current block may be signaled byencoding a motion vector difference and an indicator for a motion vectorpredictor. The motion vector difference may mean a difference betweenthe motion vector of the current block and the motion vector predictor.

The prediction unit may generate a prediction signal based on variousprediction methods and prediction techniques described below. Forexample, the prediction unit may not only apply intra prediction orinter prediction but also simultaneously apply both intra prediction andinter prediction, in order to predict the current block. A predictionmethod of simultaneously applying both intra prediction and interprediction for prediction of the current block may be called combinedinter and intra prediction (CIIP). In addition, the prediction unit mayperform intra block copy (IBC) for prediction of the current block.Intra block copy may be used for content image/video coding of a game orthe like, for example, screen content coding (SCC). IBC is a method ofpredicting a current picture using a previously reconstructed referenceblock in the current picture at a location apart from the current blockby a predetermined distance. When IBC is applied, the location of thereference block in the current picture may be encoded as a vector (blockvector) corresponding to the predetermined distance. IBC basicallyperforms prediction in the current picture, but may be performedsimilarly to inter prediction in that a reference block is derivedwithin the current picture. That is, IBC may use at least one of theinter prediction techniques described in the present disclosure.

The prediction signal generated by the prediction unit may be used togenerate a reconstructed signal or to generate a residual signal. Thesubtractor 115 may generate a residual signal (residual block orresidual sample array) by subtracting the prediction signal (predictedblock or prediction sample array) output from the prediction unit fromthe input image signal (original block or original sample array). Thegenerated residual signal may be transmitted to the transformer 120.

The transformer 120 may generate transform coefficients by applying atransform technique to the residual signal. For example, the transformtechnique may include at least one of a discrete cosine transform (DCT),a discrete sine transform (DST), a karhunen-loeve transform (KLT), agraph-based transform (GBT), or a conditionally non-linear transform(CNT). Here, the GBT means transform obtained from a graph whenrelationship information between pixels is represented by the graph. TheCNT refers to transform acquired based on a prediction signal generatedusing all previously reconstructed pixels. In addition, the transformprocess may be applied to square pixel blocks having the same size ormay be applied to blocks having a variable size rather than square.

The quantizer 130 may quantize the transform coefficients and transmitthem to the entropy encoder 190. The entropy encoder 190 may encode thequantized signal (information on the quantized transform coefficients)and output a bitstream. The information on the quantized transformcoefficients may be referred to as residual information. The quantizer130 may rearrange quantized transform coefficients in a block form intoa one-dimensional vector form based on a coefficient scanning order andgenerate information on the quantized transform coefficients based onthe quantized transform coefficients in the one-dimensional vector form.

The entropy encoder 190 may perform various encoding methods such as,for example, exponential Golomb, context-adaptive variable length coding(CAVLC), context-adaptive binary arithmetic coding (CABAC), and thelike. The entropy encoder 190 may encode information necessary forvideo/image reconstruction other than quantized transform coefficients(e.g., values of syntax elements, etc.) together or separately. Encodedinformation (e.g., encoded video/image information) may be transmittedor stored in units of network abstraction layers (NALs) in the form of abitstream. The video/image information may further include informationon various parameter sets such as an adaptation parameter set (APS), apicture parameter set (PPS), a sequence parameter set (SPS), or a videoparameter set (VPS). In addition, the video/image information mayfurther include general constraint information. The signaledinformation, transmitted information and/or syntax elements described inthe present disclosure may be encoded through the above-describedencoding procedure and included in the bitstream.

The bitstream may be transmitted over a network or may be stored in adigital storage medium. The network may include a broadcasting networkand/or a communication network, and the digital storage medium mayinclude various storage media such as USB, SD, CD, DVD, Blu-ray, HDD,SSD, and the like. A transmitter (not shown) transmitting a signaloutput from the entropy encoder 190 and/or a storage unit (not shown)storing the signal may be included as internal/external element of theimage encoding apparatus 100. Alternatively, the transmitter may beprovided as the component of the entropy encoder 190.

The quantized transform coefficients output from the quantizer 130 maybe used to generate a residual signal. For example, the residual signal(residual block or residual samples) may be reconstructed by applyingdequantization and inverse transform to the quantized transformcoefficients through the dequantizer 140 and the inverse transformer150.

The adder 155 adds the reconstructed residual signal to the predictionsignal output from the inter prediction unit 180 or the intra predictionunit 185 to generate a reconstructed signal (reconstructed picture,reconstructed block, reconstructed sample array). If there is noresidual for the block to be processed, such as a case where the skipmode is applied, the predicted block may be used as the reconstructedblock. The adder 155 may be called a reconstructor or a reconstructedblock generator. The generated reconstructed signal may be used forintra prediction of a next block to be processed in the current pictureand may be used for inter prediction of a next picture through filteringas described below.

The filter 160 may improve subjective/objective image quality byapplying filtering to the reconstructed signal. For example, the filter160 may generate a modified reconstructed picture by applying variousfiltering methods to the reconstructed picture and store the modifiedreconstructed picture in the memory 170, specifically, a DPB of thememory 170. The various filtering methods may include, for example,deblocking filtering, a sample adaptive offset, an adaptive loop filter,a bilateral filter, and the like. The filter 160 may generate variousinformation related to filtering and transmit the generated informationto the entropy encoder 190 as described later in the description of eachfiltering method. The information related to filtering may be encoded bythe entropy encoder 190 and output in the form of a bitstream.

The modified reconstructed picture transmitted to the memory 170 may beused as the reference picture in the inter prediction unit 180. Wheninter prediction is applied through the image encoding apparatus 100,prediction mismatch between the image encoding apparatus 100 and theimage decoding apparatus may be avoided and encoding efficiency may beimproved.

The DPB of the memory 170 may store the modified reconstructed picturefor use as a reference picture in the inter prediction unit 180. Thememory 170 may store the motion information of the block from which themotion information in the current picture is derived (or encoded) and/orthe motion information of the blocks in the picture that have alreadybeen reconstructed. The stored motion information may be transmitted tothe inter prediction unit 180 and used as the motion information of thespatial neighboring block or the motion information of the temporalneighboring block. The memory 170 may store reconstructed samples ofreconstructed blocks in the current picture and may transfer thereconstructed samples to the intra prediction unit 185.

Overview of Image Decoding Apparatus

FIG. 3 is a view schematically showing an image decoding apparatus, towhich an embodiment of the present disclosure is applicable.

As shown in FIG. 3, the image decoding apparatus 200 may include anentropy decoder 210, a dequantizer 220, an inverse transformer 230, anadder 235, a filter 240, a memory 250, an inter prediction unit 260 andan intra prediction unit 265. The inter prediction unit 260 and theintra prediction unit 265 may be collectively referred to as a“prediction unit”. The dequantizer 220 and the inverse transformer 230may be included in a residual processor.

All or at least some of a plurality of components configuring the imagedecoding apparatus 200 may be configured by a hardware component (e.g.,a decoder or a processor) according to an embodiment. In addition, thememory 250 may include a decoded picture buffer (DPB) or may beconfigured by a digital storage medium.

The image decoding apparatus 200, which has received a bitstreamincluding video/image information, may reconstruct an image byperforming a process corresponding to a process performed by the imageencoding apparatus 100 of FIG. 2. For example, the image decodingapparatus 200 may perform decoding using a processing unit applied inthe image encoding apparatus. Thus, the processing unit of decoding maybe a coding unit, for example. The coding unit may be acquired bypartitioning a coding tree unit or a largest coding unit. Thereconstructed image signal decoded and output through the image decodingapparatus 200 may be reproduced through a reproducing apparatus (notshown).

The image decoding apparatus 200 may receive a signal output from theimage encoding apparatus of FIG. 2 in the form of a bitstream. Thereceived signal may be decoded through the entropy decoder 210. Forexample, the entropy decoder 210 may parse the bitstream to deriveinformation (e.g., video/image information) necessary for imagereconstruction (or picture reconstruction). The video/image informationmay further include information on various parameter sets such as anadaptation parameter set (APS), a picture parameter set (PPS), asequence parameter set (SPS), or a video parameter set (VPS). Inaddition, the video/image information may further include generalconstraint information. The image decoding apparatus may further decodepicture based on the information on the parameter set and/or the generalconstraint information. Signaled/received information and/or syntaxelements described in the present disclosure may be decoded through thedecoding procedure and obtained from the bitstream. For example, theentropy decoder 210 decodes the information in the bitstream based on acoding method such as exponential Golomb coding, CAVLC, or CABAC, andoutput values of syntax elements required for image reconstruction andquantized values of transform coefficients for residual. Morespecifically, the CABAC entropy decoding method may receive a bincorresponding to each syntax element in the bitstream, determine acontext model using a decoding target syntax element information,decoding information of a neighboring block and a decoding target blockor information of a symbol/bin decoded in a previous stage, and performarithmetic decoding on the bin by predicting a probability of occurrenceof a bin according to the determined context model, and generate asymbol corresponding to the value of each syntax element. In this case,the CABAC entropy decoding method may update the context model by usingthe information of the decoded symbol/bin for a context model of a nextsymbol/bin after determining the context model. The information relatedto the prediction among the information decoded by the entropy decoder210 may be provided to the prediction unit (the inter prediction unit260 and the intra prediction unit 265), and the residual value on whichthe entropy decoding was performed in the entropy decoder 210, that is,the quantized transform coefficients and related parameter information,may be input to the dequantizer 220. In addition, information onfiltering among information decoded by the entropy decoder 210 may beprovided to the filter 240. Meanwhile, a receiver (not shown) forreceiving a signal output from the image encoding apparatus may befurther configured as an internal/external element of the image decodingapparatus 200, or the receiver may be a component of the entropy decoder210.

Meanwhile, the image decoding apparatus according to the presentdisclosure may be referred to as a video/image/picture decodingapparatus. The image decoding apparatus may be classified into aninformation decoder (video/image/picture information decoder) and asample decoder (video/image/picture sample decoder). The informationdecoder may include the entropy decoder 210. The sample decoder mayinclude at least one of the dequantizer 220, the inverse transformer230, the adder 235, the filter 240, the memory 250, the inter predictionunit 260 or the intra prediction unit 265.

The dequantizer 220 may dequantize the quantized transform coefficientsand output the transform coefficients. The dequantizer 220 may rearrangethe quantized transform coefficients in the form of a two-dimensionalblock. In this case, the rearrangement may be performed based on thecoefficient scanning order performed in the image encoding apparatus.The dequantizer 220 may perform dequantization on the quantizedtransform coefficients by using a quantization parameter (e.g.,quantization step size information) and obtain transform coefficients.

The inverse transformer 230 may inversely transform the transformcoefficients to obtain a residual signal (residual block, residualsample array).

The prediction unit may perform prediction on the current block andgenerate a predicted block including prediction samples for the currentblock. The prediction unit may determine whether intra prediction orinter prediction is applied to the current block based on theinformation on the prediction output from the entropy decoder 210 andmay determine a specific intra/inter prediction mode (predictiontechnique).

It is the same as described in the prediction unit of the image encodingapparatus 100 that the prediction unit may generate the predictionsignal based on various prediction methods (techniques) which will bedescribed later.

The intra prediction unit 265 may predict the current block by referringto the samples in the current picture. The description of the intraprediction unit 185 is equally applied to the intra prediction unit 265.

The inter prediction unit 260 may derive a predicted block for thecurrent block based on a reference block (reference sample array)specified by a motion vector on a reference picture. In this case, inorder to reduce the amount of motion information transmitted in theinter prediction mode, motion information may be predicted in units ofblocks, subblocks, or samples based on correlation of motion informationbetween the neighboring block and the current block. The motioninformation may include a motion vector and a reference picture index.The motion information may further include inter prediction direction(L0 prediction, L1 prediction, Bi prediction, etc.) information. In thecase of inter prediction, the neighboring block may include a spatialneighboring block present in the current picture and a temporalneighboring block present in the reference picture. For example, theinter prediction unit 260 may configure a motion information candidatelist based on neighboring blocks and derive a motion vector of thecurrent block and/or a reference picture index based on the receivedcandidate selection information. Inter prediction may be performed basedon various prediction modes, and the information on the prediction mayinclude information indicating a mode of inter prediction for thecurrent block.

The adder 235 may generate a reconstructed signal (reconstructedpicture, reconstructed block, reconstructed sample array) by adding theobtained residual signal to the prediction signal (predicted block,predicted sample array) output from the prediction unit (including theinter prediction unit 260 and/or the intra prediction unit 265). Ifthere is no residual for the block to be processed, such as when theskip mode is applied, the predicted block may be used as thereconstructed block. The description of the adder 155 is equallyapplicable to the adder 235. The adder 235 may be called a reconstructoror a reconstructed block generator. The generated reconstructed signalmay be used for intra prediction of a next block to be processed in thecurrent picture and may be used for inter prediction of a next picturethrough filtering as described below.

The filter 240 may improve subjective/objective image quality byapplying filtering to the reconstructed signal. For example, the filter240 may generate a modified reconstructed picture by applying variousfiltering methods to the reconstructed picture and store the modifiedreconstructed picture in the memory 250, specifically, a DPB of thememory 250. The various filtering methods may include, for example,deblocking filtering, a sample adaptive offset, an adaptive loop filter,a bilateral filter, and the like.

The (modified) reconstructed picture stored in the DPB of the memory 250may be used as a reference picture in the inter prediction unit 260. Thememory 250 may store the motion information of the block from which themotion information in the current picture is derived (or decoded) and/orthe motion information of the blocks in the picture that have alreadybeen reconstructed. The stored motion information may be transmitted tothe inter prediction unit 260 so as to be utilized as the motioninformation of the spatial neighboring block or the motion informationof the temporal neighboring block. The memory 250 may storereconstructed samples of reconstructed blocks in the current picture andtransfer the reconstructed samples to the intra prediction unit 265.

In the present disclosure, the embodiments described in the filter 160,the inter prediction unit 180, and the intra prediction unit 185 of theimage encoding apparatus 100 may be equally or correspondingly appliedto the filter 240, the inter prediction unit 260, and the intraprediction unit 265 of the image decoding apparatus 200.

Overview of Image Partitioning

The video/image coding method according to the present disclosure may beperformed based on an image partitioning structure as follows.Specifically, the procedures of prediction, residual processing((inverse) transform, (de)quantization, etc.), syntax element coding,and filtering, which will be described later, may be performed based ona CTU, CU (and/or TU, PU) derived based on the image partitioningstructure. The image may be partitioned in block units and the blockpartitioning procedure may be performed in the image partitioner 110 ofthe encoding apparatus. The partitioning related information may beencoded by the entropy encoder 190 and transmitted to the image decodingapparatus in the form of a bitstream. The entropy decoder 210 of theimage decoding apparatus may derive a block partitioning structure ofthe current picture based on the partitioning related informationobtained from the bitstream, and based on this, may perform a series ofprocedures (e.g., prediction, residual processing, block/picturereconstruction, in-loop filtering, etc.) for image decoding.

Pictures may be partitioned into a sequence of coding tree units (CTUs).FIG. 4 shows an example in which a picture is partitioned into CTUs. TheCTU may correspond to a coding tree block (CTB). Alternatively, the CTUmay include a coding tree block of luma samples and two coding treeblocks of corresponding chroma samples. For example, for a picture thatcontains three sample arrays, the CTU may include an N×N block of lumasamples and two corresponding blocks of chroma samples.

Overview of Partitioning of CTU

As described above, the coding unit may be acquired by recursivelypartitioning the coding tree unit (CTU) or the largest coding unit (LCU)according to a quad-tree/binary-tree/ternary-tree (QT/BT/TT) structure.For example, the CTU may be first partitioned into quadtree structures.Thereafter, leaf nodes of the quadtree structure may be furtherpartitioned by a multi-type tree structure.

Partitioning according to quadtree means that a current CU (or CTU) ispartitioned into equally four. By partitioning according to quadtree,the current CU may be partitioned into four CUs having the same widthand the same height. When the current CU is no longer partitioned intothe quadtree structure, the current CU corresponds to the leaf node ofthe quad-tree structure. The CU corresponding to the leaf node of thequadtree structure may be no longer partitioned and may be used as theabove-described final coding unit. Alternatively, the CU correspondingto the leaf node of the quadtree structure may be further partitioned bya multi-type tree structure.

FIG. 5 is a view showing an embodiment of a splitting type of a blockaccording to a multi-type tree structure. Splitting according to themulti-type tree structure may include two types of splitting accordingto a binary tree structure and two types of splitting according to aternary tree structure.

The two types of splitting according to the binary tree structure mayinclude vertical binary splitting (SPLIT_BT_VER) and horizontal binarysplitting (SPLIT_BT_HOR). Vertical binary splitting (SPLIT_BT_VER) meansthat the current CU is split into equally two in the vertical direction.As shown in FIG. 4, by vertical binary splitting, two CUs having thesame height as the current CU and having a width which is half the widthof the current CU may be generated. Horizontal binary splitting(SPLIT_BT_HOR) means that the current CU is split into equally two inthe horizontal direction. As shown in FIG. 5, by horizontal binarysplitting, two CUs having a height which is half the height of thecurrent CU and having the same width as the current CU may be generated.

Two types of splitting according to the ternary tree structure mayinclude vertical ternary splitting (SPLIT_TT_VER) and horizontal ternarysplitting (SPLIT_TT_HOR). In vertical ternary splitting (SPLIT_TT_VER),the current CU is split in the vertical direction at a ratio of 1:2:1.As shown in FIG. 5, by vertical ternary splitting, two CUs having thesame height as the current CU and having a width which is ¼ of the widthof the current CU and a CU having the same height as the current CU andhaving a width which is half the width of the current CU may begenerated. In horizontal ternary splitting (SPLIT_TT_HOR), the currentCU is split in the horizontal direction at a ratio of 1:2:1. As shown inFIG. 4, by horizontal ternary splitting, two CUs having a height whichis ¼ of the height of the current CU and having the same width as thecurrent CU and a CU having a height which is half the height of thecurrent CU and having the same width as the current CU may be generated.

FIG. 6 is a view showing a signaling mechanism of block splittinginformation in a structure of a quadtree with nested multi-type treeaccording to the present disclosure.

Here, the CTU is treated as the root node of the quadtree, and ispartitioned for the first time into a quadtree structure. Information(e.g., qt_split_flag) indicating whether quadtree splitting is performedwith respect to the current CU (CTU or node (QT_node) of the quadtree)is signaled. For example, when the qt_split_flag has a first value(e.g., “1”), the current CU may be quadtree-partitioned. In addition,when qt_split_flag has a second value (e.g., “0”), the current CU is notquadtree-partitioned, but becomes the leaf node (QT_leaf_node) of thequadtree. Each quadtree leaf node may then be further partitioned intomultitype tree structures. That is, the leaf node of the quadtree maybecome the node (MTT_node) of the multi-type tree. In the multitype treestructure, a first flag (e.g., Mtt_split_cu_flag) is signaled toindicate whether the current node is additionally partitioned. If thecorresponding node is additionally partitioned (e.g., if the first flagis 1), a second flag (e.g., Mtt_split_cu_vertical_flag) may be signaledto indicate the splitting direction. For example, the splittingdirection may be a vertical direction if the second flag is 1 and may bea horizontal direction if the second flag is 0. Then, a third flag(e.g., Mtt_split_cu_binary_flag) may be signaled to indicate whether thesplit type is a binary split type or a ternary split type. For example,the split type may be a binary split type when the third flag is 1 andmay be a ternary split type when the third flag is 0. The node of themulti-type tree acquired by binary splitting or ternary splitting may befurther partitioned into multi-type tree structures. However, the nodeof the multi-type tree may not be partitioned into quadtree structures.If the first flag is 0, the corresponding node of the multi-type tree isno longer split but becomes the leaf node (MTT_leaf_node) of themulti-type tree. The CU corresponding to the leaf node of the multi-typetree may be used as the above-described final coding unit.

Based on the mtt_split_cu_vertical_flag and themtt_split_cu_binary_flag, a multi-type tree splitting mode(MttSplitMode) of a CU may be derived as shown in Table 1 below. In thefollowing description, the multi-tree splitting mode may be referred toas a multi-tree split type or a split type for short.

TABLE 1 MttSplitMode mtt_split_cu_vertical_flag mtt_split_cu_binary_flagSPLIT_TT_HOR 0 0 SPLIT_BT_HOR 0 1 SPLIT_TT_VER 1 0 SPLIT_BT_VER 1 1

FIG. 7 is a view showing an example in which a CTU is partitioned intomultiple CUs by applying a multi-type tree after applying a quadtree. InFIG. 7, bold block edges 710 represent quadtree partitioning and theremaining edges 720 represent multitype tree partitioning. The CU maycorrespond to a coding block (CB). In an embodiment, the CU may includea coding block of luma samples and two coding blocks of chroma samplescorresponding to the luma samples. A chroma component (sample) CB or TBsize may be derived based on a luma component (sample) CB or TB sizeaccording to the component ratio according to the color format (chromaformat, e.g., 4:4:4, 4:2:2, 4:2:0 or the like) of the picture/image. Incase of 4:4:4 color format, the chroma component CB/TB size may be setequal to be luma component CB/TB size. In case of 4:2:2 color format,the width of the chroma component CB/TB may be set to half the width ofthe luma component CB/TB and the height of the chroma component CB/TBmay be set to the height of the luma component CB/TB. In case of 4:2:0color format, the width of the chroma component CB/TB may be set to halfthe width of the luma component CB/TB and the height of the chromacomponent CB/TB may be set to half the height of the luma componentCB/TB.

In an embodiment, when the size of the CTU is 128 based on the lumasample unit, the size of the CU may have a size from 128×128 to 4×4which is the same size as the CTU. In one embodiment, in case of 4:2:0color format (or chroma format), a chroma CB size may have a size from64×64 to 2×2.

Meanwhile, in an embodiment, the CU size and the TU size may be thesame. Alternatively, there may be a plurality of TUs in a CU region. TheTU size generally represents a luma component (sample) transform block(TB) size.

The TU size may be derived based a largest allowable TB size maxTbSizewhich is a predetermined value. For example, when the CU size is greaterthan maxTbSize, a plurality of TUs (TBs) having maxTbSize may be derivedfrom the CU and transform/inverse transform may be performed in units ofTU (TB). For example, the largest allowable luma TB size may be 64×64and the largest allowable chroma TB size may be 32×32. If the width orheight of the CB partitioned according to the tree structure is largerthan the largest transform width or height, the CB may be automatically(or implicitly) partitioned until the TB size limit in the horizontaland vertical directions is satisfied.

In addition, for example, when intra prediction is applied, an intraprediction mode/type may be derived in units of CU (or CB) and aneighboring reference sample derivation and prediction sample generationprocedure may be performed in units of TU (or TB). In this case, theremay be one or a plurality of TUs (or TBs) in one CU (or CB) region and,in this case, the plurality of TUs or (TBs) may share the same intraprediction mode/type.

Meanwhile, for a quadtree coding tree scheme with nested multitype tree,the following parameters may be signaled as SPS syntax elements from theimage encoding apparatus to the decoding apparatus. For example, atleast one of a CTU size which is a parameter representing the root nodesize of a quadtree, MinQTSize which is a parameter representing theminimum allowed quadtree leaf node size, MaxBtSize which is a parameterrepresenting the maximum allowed binary tree root node size, MaxTtSizewhich is a parameter representing the maximum allowed ternary tree rootnode size, MaxMttDepth which is a parameter representing the maximumallowed hierarchy depth of multi-type tree splitting from a quadtreeleaf node, MinBtSize which is a parameter representing the minimumallowed binary tree leaf node size, or MinTtSize which is a parameterrepresenting the minimum allowed ternary tree leaf node size issignaled.

As an embodiment of using 4:2:0 chroma format, the CTU size may be setto 128×128 luma blocks and two 64×64 chroma blocks corresponding to theluma blocks. In this case, MinOTSize may be set to 16×16, MaxBtSize maybe set to 128×128, MaxTtSzie may be set to 64×64, MinBtSize andMinTtSize may be set to 4×4, and MaxMttDepth may be set to 4. Quadtreepartitioning may be applied to the CTU to generate quadtree leaf nodes.The quadtree leaf node may be called a leaf QT node. Quadtree leaf nodesmay have a size from a 16×16 size (e.g., the MinOTSize) to a 128×128size (e.g., the CTU size). If the leaf QT node is 128×128, it may not beadditionally partitioned into a binary tree/ternary tree. This isbecause, in this case, even if partitioned, it exceeds MaxBtsize andMaxTtszie (e.g., 64×64). In other cases, leaf QT nodes may be furtherpartitioned into a multitype tree. Therefore, the leaf QT node is theroot node for the multitype tree, and the leaf QT node may have amultitype tree depth (mttDepth) 0 value. If the multitype tree depthreaches MaxMttdepth (e.g., 4), further partitioning may not beconsidered further. If the width of the multitype tree node is equal toMinBtSize and less than or equal to 2×MinTtSize, then no furtherhorizontal partitioning may be considered. If the height of themultitype tree node is equal to MinBtSize and less than or equal to2×MinTtSize, no further vertical partitioning may be considered. Whenpartitioning is not considered, the image encoding apparatus may skipsignaling of partitioning information. In this case, the image decodingapparatus may derive partitioning information with a predeterminedvalue.

Meanwhile, one CTU may include a coding block of luma samples(hereinafter referred to as a “luma block”) and two coding blocks ofchroma samples corresponding thereto (hereinafter referred to as “chromablocks”). The above-described coding tree scheme may be equally orseparately applied to the luma block and chroma block of the current CU.Specifically, the luma and chroma blocks in one CTU may be partitionedinto the same block tree structure and, in this case, the tree structureis represented as SINGLE_TREE. Alternatively, the luma and chroma blocksin one CTU may be partitioned into separate block tree structures, and,in this case, the tree structure may be represented as DUAL_TREE. Thatis, when the CTU is partitioned into dual trees, the block treestructure for the luma block and the block tree structure for the chromablock may be separately present. In this case, the block tree structurefor the luma block may be called DUAL_TREE_LUMA, and the block treestructure for the chroma component may be called DUAL_TREE_CHROMA. For Pand B slice/tile groups, luma and chroma blocks in one CTU may belimited to have the same coding tree structure. However, for Islice/tile groups, luma and chroma blocks may have a separate block treestructure from each other. If the separate block tree structure isapplied, the luma CTB may be partitioned into CUs based on a particularcoding tree structure, and the chroma CTB may be partitioned into chromaCUs based on another coding tree structure. That is, this means that aCU in an I slice/tile group, to which the separate block tree structureis applied, may include a coding block of luma components or codingblocks of two chroma components and a CU of a P or B slice/tile groupmay include blocks of three color components (a luma component and twochroma components).

Although a quadtree coding tree structure with a nested multitype treehas been described, a structure in which a CU is partitioned is notlimited thereto. For example, the BT structure and the TT structure maybe interpreted as a concept included in a multiple partitioning tree(MPT) structure, and the CU may be interpreted as being partitionedthrough the QT structure and the MPT structure. In an example where theCU is partitioned through a QT structure and an MPT structure, a syntaxelement (e.g., MPT_split_type) including information on how many blocksthe leaf node of the QT structure is partitioned into and a syntaxelement (e.g., MPT_split_mode) including information on which ofvertical and horizontal directions the leaf node of the QT structure ispartitioned into may be signaled to determine a partitioning structure.

In another example, the CU may be partitioned in a different way thanthe QT structure, BT structure or TT structure. That is, unlike that theCU of the lower depth is partitioned into ¼ of the CU of the higherdepth according to the QT structure, the CU of the lower depth ispartitioned into ½ of the CU of the higher depth according to the BTstructure, or the CU of the lower depth is partitioned into ¼ or ½ ofthe CU of the higher depth according to the TT structure, the CU of thelower depth may be partitioned into ⅕, ⅓, ⅜, ⅗, ⅔, or ⅝ of the CU of thehigher depth in some cases, and the method of partitioning the CU is notlimited thereto.

The quadtree coding block structure with the multi-type tree may providea very flexible block partitioning structure. Because of the partitiontypes supported in a multi-type tree, different partition patterns maypotentially result in the same coding block structure in some cases. Inthe image encoding apparatus and the decoding apparatus, by limiting theoccurrence of such redundant partition patterns, a data amount ofpartitioning information may be reduced.

In addition, in encoding and decoding of a video/image according to thepresent document, an image processing basis may have a hierarchicalstructure. One picture may be divided into one or more tiles, bricks,slices, and/or tile groups. One slice may include one or more bricks.One brick may include one or more CTU rows within a tile. A slice mayinclude bricks of a picture, wherein the number of the bricks is aninteger. One tile group may include one or more tiles. One tile mayinclude one or more CTUs. The CTU may be partitioned into one or moreCUs. A tile may be a quadrangular area composed of particular tile rowsand particular tile columns composed of a plurality of CTUs within apicture. A tile group may include tiles according to tile rasterscanning within a picture, wherein the number of the tiles is aninteger. A slice header may carry information/a parameter applicable tothe corresponding slice (blocks in the slice). When the encodingapparatus or the decoding apparatus has a multi-core processor,encoding/decoding procedures for the tile, slice, brick, and/or tilegroup may be performed in parallel.

In the present disclosure, the names or concepts of a slice or a tilegroup may be used interchangeably. That is, a tile group header may bereferred to as a slice header. Herein, a slice may have one of slicetypes including an intra (I) slice, a predictive (P) slice, and abi-predictive (B) slice. For blocks within an I slice, inter predictionis not used for prediction, and only intra prediction may be used. Evenin this case, an original sample value may be coded and signaled withoutprediction. For blocks within a P slice, intra prediction or interprediction may be used. When inter prediction is used, onlyuni-prediction may be used. In the meantime, for blocks within a Bslice, intra prediction or inter prediction may be used. When interprediction is used, at most bi-prediction may be used.

According to a characteristic (for example, resolution) of a video imageor considering coding efficiency or parallel processing, the encodingapparatus may determine a tile/tile group, a brick, a slice, and largestand smallest coding unit sizes. In addition, information on this orinformation for deriving this may be included in a bitstream.

The decoding apparatus may obtain information indicating whether a CTUwithin a tile/tile group, a brick, a slice, or a tile of a currentpicture is partitioned into multiple coding units. The encodingapparatus and the decoding apparatus signal such information only undera particular condition, thereby increasing encoding efficiency.

The slice header (slice header syntax) may include information/aparameter applicable to the slice in common. APS (APS syntax) or PPS(PPS syntax) may include information/a parameter applicable to one ormore pictures in common. SPS (SPS syntax) may include information/aparameter applicable to one or more sequences in common. VPS (VPSsyntax) may include information/a parameter applicable to multiplelayers in common. DPS (DPS syntax) may include information/a parameterapplicable to the entire video in common. DPS may include information/aparameter related to combination of coded video sequences (CVSs).

In addition, for example, information on the partitioning andconfiguration of the tile/tile group/brick/slice may be constructed atthe encoding stage through the high level syntax, and transmitted in theform of a bitstream to the decoding apparatus.

Quantization/Dequantization

As described above, the quantizer of the encoding apparatus may derivequantized transformation(transform) coefficients by applyingquantization to transformation coefficients(transform coefficients). Thedequantizer of the encoding apparatus or the dequantizer of the decodingapparatus may derive transformation coefficients by applyingdequantization to quantized transformation coefficients.

In encoding and decoding of a video/still image, a quantization ratiomay be changed, and the changed quantization ratio may be used to adjusta compression ratio. From an angle on realization, consideringcomplexity, a quantization parameter (QP) may be used instead of using aquantization ratio directly. For example, quantization parameters havinginteger values of 0 to 63 may be used, and each quantization parametervalue may correspond to an actual quantization ratio. In addition, aquantization parameter QP_(Y) for a luma component (luma sample) and aquantization parameter QP_(C) for a chroma component (chroma sample) maybe set to be different from each other.

In a quantization process, a transformation coefficient C may be inputand divided by a quantization ratio Qstep, and on the basis of this, aquantized transformation coefficient C′ may be obtained. In this case,considering calculation complexity, a quantization ratio may bemultiplied by a scale to be in the form of an integer, and shiftoperation may be performed by the value corresponding to the scalevalue. Based on multiplication of a quantization ratio and a scalevalue, a quantization scale may be derived. That is, the quantizationscale may be derived according to a QP. The quantization scale may beapplied to the transformation coefficient C, and on the basis of this,the quantized transformation coefficient C′ may be derived.

The dequantization process is the reverse process of the quantizationprocess. A quantized transformation coefficient C′ may be multiplied bya quantization ratio Qstep, and on the basis of this, a reconstructedtransformation coefficient C″ may be obtained. In this case, a levelscale may be derived according to the quantization parameter. The levelscale may be applied to the quantized transformation coefficient C′, andon the basis of this, a reconstructed transformation coefficient C″ maybe derived. The reconstructed transformation coefficient C″ may beslightly different from the original transformation coefficient Cbecause of the loss in a transformation and/or quantization process.Therefore, the encoding apparatus may perform dequantization in the samemanner as the decoding apparatus.

In the meantime, an adaptive frequency weighting quantization technologyin which a quantization strength is adjusted according to frequency maybe applied. The adaptive frequency weighting quantization technology isa method of applying quantization strengths that vary from frequency tofrequency. In the adaptive frequency weighting quantization,quantization strengths varying from frequency to frequency may beapplied by using a pre-defined quantization scaling matrix. That is, theabove-described quantization/dequantization process may be performedfurther on the basis of the quantization scaling matrix. For example,different quantization scaling matrices may be used according to a sizeof a current block and/or whether the prediction mode applied to thecurrent block to generate a residual signal of the current block isinter prediction or intra prediction. The quantization scaling matrixmay be referred to as a quantization matrix or a scaling matrix. Thequantization scaling matrix may be pre-defined. In addition, forfrequency adaptive scaling, frequency-specific quantization scaleinformation for the quantization scaling matrix may beconstructed/encoded by the encoding apparatus, and may be signaled tothe decoding apparatus. The frequency-specific quantization scaleinformation may be referred to as quantization scaling information. Thefrequency-specific quantization scale information may include scalinglist data (scaling_list_data). The (modified) quantization scalingmatrix may be derived on the basis of the scaling list data. Inaddition, the frequency-specific quantization scale information mayinclude a present flag information indicating whether the scaling listdata is present. Alternatively, when the scaling list data is signaledat a high level (e.g., an SPS), further included is informationindicating whether the scaling list data is modified at a lower level(e.g., a PPS or a tile group header, etc.).

Transformation/Inverse Transformation

As described above, the encoding apparatus may derive a residual block(residual samples) on the basis of a block (prediction samples)predicted through intra/inter/IBC prediction, and may derive quantizedtransformation coefficients by applying transformation and quantizationto the derived residual samples. Being included in residual codingsyntax, information (residual information) on quantized transformationcoefficients may be encoded and output in the form of a bitstream. Thedecoding apparatus may obtain information (residual information) on thequantized transformation coefficients from the bitstream, and may derivethe quantized transformation coefficients by performing decoding. Thedecoding apparatus may derive residual samples throughdequantization/inverse transformation on the basis of the quantizedtransformation coefficients. As described above, either thequantization/dequantization or the transformation/inverse transformationor both may be omitted. When the transformation/inverse transformationis omitted, the transformation coefficient may be referred to as acoefficient or a residual coefficient, or may still be referred to as atransformation coefficient for consistency of expression. Whether thetransformation/inverse transformation is omitted may be signaled on thebasis of a transformation skip flag(e.g., transform_skip_flag). A firstvalue (e.g., 0) of the transform_skip_flag may indicate that whethertransformation is omitted is determined by another syntax element. Asecond value (e.g., 1) of the transform_skip_flag may indicatetransformation omission (e.g., skip).

The transformation/inverse transformation may be performed on the basisof a transformation kernel(s). For example, a multiple transformselection (MTS) scheme for performing transformation/inversetransformation may be applied. In this case, some of multipletransformation kernel sets may be selected and applied to a currentblock. A transformation kernel may be referred to as various terms, suchas a transformation matrix, a transformation type, etc. For example, atransformation kernel set may refer to a combination of avertical-direction transformation kernel (vertical transformationkernel) and a horizontal-direction transformation kernel (horizontaltransformation kernel).

The transformation/inverse transformation may be performed on a per CUor TU basis. That is, the transformation/inverse transformation may beapplied to residual samples in a CU or residual samples in a TU. A CUsize and a TU size may be the same, or a plurality of TUs may be presentin a CU area. In the meantime, a CU size may generally refer to a lumacomponent (sample) CB size. A TU size may generally refer to a lumacomponent (sample) TB size. A chroma component (sample) CB or TB sizemay be derived on the basis of a luma component (sample) CB or TB sizeaccording to a component ratio according to a color format (a chromaformat, e.g., 4:4:4, 4:2:2, 4:2:0, or the like). The TU size may bederived on the basis of maxTbSize. For example, when the CU size isgreater than the maxTbSize, a plurality of TUs (TBs) having themaxTbSize may be derived from the CU and transformation/inversetransformation may be performed on a per TU (TB) basis. The maxTbSizemay be considered in determining whether various intra prediction types,such as ISP, are applied. Information on the maxTbSize may bepre-determined. Alternatively, information on the maxTbSize may begenerated and encoded by the encoding apparatus and signaled to theencoding apparatus.

Entropy Coding

As described above with reference to FIG. 2, some or all of video/imageinformation may be entropy-encoded by the entropy encoder 190. Some orall of video/image information described with reference to FIG. 3 may beentropy-decoded by the entropy decoder 310. In this case, thevideo/image information may be encoded/decoded on a per syntax elementbasis. In the present document, encoding/decoding of information mayinclude encoding/decoding by the method described in this paragraph.

FIG. 8 shows a block diagram of CABAC for encoding one syntax element.In an encoding process of CABAC, first, when an input signal is a syntaxelement not a binary value, the input signal is transformed into abinary value through binarization. When an input signal is already abinary value, the input signal bypasses binarization. Herein, eachbinary number 0 or 1 constituting a binary value may be referred to as abin. For example, a binary string (bin string) after binarization is110, each of 1, 1, and 0 is referred to as one bin. The bin(s) for onesyntax element may refer to a value of the syntax element.

Binarized bins may be input to a regular coding engine or a bypasscoding engine. The regular coding engine may assign a context model thatapplies a probability value to a corresponding bin, and may encode thebin on the basis of the assigned context model. After performing codingon each bin, the regular coding engine may update a probability modelfor the bin. The bins coded in this way may be referred to ascontext-coded bins. The bypass coding engine may omit a procedure forestimating a probability of an input bin and a procedure for updatingthe probability model applied to the bin after coding. In the case ofthe bypass coding engine, an input bin is coded by applying a uniformprobability distribution (e.g., 50:50) instead of assigning a context,thereby improving a coding rate. The bins coded in this way may bereferred to as bypass bins. The context model may be assigned andupdated for each bin to be context-coded (regular-coded), and thecontext model may be indicated on the basis of ctxidx or ctxInc. ctxidxmay be derived on the basis of ctxInc. Specifically, for example, thecontext index (ctxidx) indicating a context model for each of theregular-coded bins may be derived as the sum of a context indexincrement (ctxInc) and a context index offset (ctxIdxOffset). Herein,ctxInc varying from bin to bin may be derived. The ctxIdxOffset may berepresented by the lowest value of the ctxIdx. The lowest value of thectxIdx may be referred to as an initial value (initValue) of the ctxIdx.The ctxIdxOffset is a value generally used for distinguishment fromcontext models for other syntax elements, and a context model for onesyntax element may be distinguished/derived on the basis of ctxinc.

In an entropy encoding procedure, it is determined whether to performencoding through the regular coding engine or perform encoding throughthe bypass coding engine, and a coding path may be switched. Entropydecoding may perform the same process as entropy encoding in reverseorder.

The above-described entropy coding may be performed as in FIGS. 9 and10, for example. Referring to FIGS. 9 and 10, the encoding apparatus(entropy encoder) may perform an entropy coding procedure on image/videoinformation. The image/video information may includepartitioning-related information, prediction-related information (e.g.,inter/intra prediction classification information, intra prediction modeinformation, and inter prediction mode information), residualinformation, and in-loop filtering-related information, or may includevarious syntax elements related thereto. The entropy coding may beperformed on a per syntax element basis. Steps S910 to S920 of FIG. 9may be performed by the entropy encoder 190 of the encoding apparatus ofFIG. 2 described above.

The encoding apparatus may perform binarization on a target syntaxelement in step S910. Herein, the binarization may be based on variousbinarization methods, such as truncated rice binarization process andfixed-length binarization process, and a binarization method for atarget syntax element may be pre-defined. The binarization procedure maybe performed by a binarization unit 191 in the entropy encoder 190.

The encoding apparatus may perform entropy encoding on the target syntaxelement in step S920. The encoding apparatus may perform regularcoding-based (context-based) or bypass coding-based encoding on a binstring of a target syntax element on the basis of an entropy codingscheme such as context-adaptive arithmetic coding (CABAC) orcontext-adaptive variable length coding (CAVLC). The output may beincluded in a bitstream. The entropy encoding procedure may be performedby an entropy encoding processor 192 in the entropy encoder 190. Asdescribed above, the bitstream may be transmitted to the decodingapparatus through a (digital) storage medium or a network.

Referring to FIGS. 11 and 12, the decoding apparatus (entropy decoder)may decode encoded image/video information. The image/video informationmay include partitioning-related information, prediction-relatedinformation (e.g., inter/intra prediction classification information,intra prediction mode information, and inter prediction modeinformation), residual information, and in-loop filtering-relatedinformation, or may include various syntax elements related thereto. Theentropy coding may be performed on a per syntax element basis. StepsS1110 to S1120 may be performed by the entropy decoder 210 of thedecoding apparatus of FIG. 3 described above.

The decoding apparatus may perform binarization on a target syntaxelement in step S1110. Herein, the binarization may be based on variousbinarization methods, such as truncated rice binarization process andfixed-length binarization process, and a binarization method for atarget syntax element may be pre-defined. The decoding apparatus mayderive available bin strings (bin string candidates) for availablevalues of a target syntax element through the binarization procedure.The binarization procedure may be performed by a binarization unit 211in the entropy decoder 210.

The decoding apparatus may perform entropy decoding on the target syntaxelement in step S1120. While the decoding apparatus sequentially decodesand parses each bin for the target syntax element from an input bit(s)in a bitstream, and may compare a derived bin string with available binstrings for the syntax element. When the derived bin string is the sameas one of the available bin strings, a value corresponding to the binstring may be derived as a value of the syntax element. If not, the nextbit in the bitstream is further parsed and the above-described procedureis performed again. Through this process, a start bit or an end bit forparticular information (particular syntax element) in a bitstream is notused, but variable-length bits are used to signal the information.Through this, relatively fewer bits are assigned for a low value,thereby increasing overall coding efficiency.

The decoding apparatus may perform context-based or bypass-baseddecoding on each bin in the bin string from a bitstream on the basis ofthe entropy coding scheme such as CABAC or CAVLC. The entropy decodingprocedure may be performed by an entropy decoding processor 212 in theentropy decoder 210. The bitstream may include various types ofinformation for image/video decoding as described above. As describedabove, the bitstream may be transmitted to the decoding apparatusthrough a (digital) storage medium or a network.

In the present document, a table (syntax table) including syntaxelements may be used to represent signaling of information from theencoding apparatus to the decoding apparatus. The order of syntaxelements in the table including the syntax elements used in the presentdocument may refer to the parsing order of syntax elements from abitstream. The encoding apparatus may construct and encode a syntaxtable so that the syntax elements are parsed in the parsing order by thedecoding apparatus. The decoding apparatus may parse and decode thesyntax elements of the syntax table from a bitstream in the parsingorder, and may thus obtain values of the syntax elements.

General Image/Video Coding Procedure

In image/video coding, pictures constituting an image/video may beencoded/decoded according to decoding order in a series. The pictureorder corresponding to the output order of decoded pictures may be setto be different from the decoding order, and on the basis of this,forward prediction as well as backward direction may be performed wheninter prediction is performed.

FIG. 13 shows an example of a schematic picture decoding procedure towhich an embodiment(s) of the present document is applicable. In FIG.13, step S1310 may be performed by the entropy decoder 210 of thedecoding apparatus described above with reference to FIG. 3. Step S1320may be performed by the prediction unit including the intra predictionunit 265 and the inter prediction unit 260. Step S1330 may be performedby the residual processor including the dequantizer 220 and the inversetransformer 230. Step S1340 may be performed by the adder 235. StepS1350 may be performed by the filter 240. Step S1310 may include theinformation decoding procedure described in the present document. StepS1320 may include the inter/intra prediction procedure described in thepresent document. Step S1330 may include the residual processingprocedure described in the present document. Step S1340 may include theblock/picture reconstruction procedure described in the presentdocument. Step S1350 may include the in-loop filtering proceduredescribed in the present document.

Referring to FIG. 13, the picture decoding procedure may schematicallyinclude, as described above with reference to FIG. 3, an image/videoinformation acquisition procedure from a bitstream (through decoding) instep S1310, the picture reconstruction procedure in steps S1320 toS1340, and the in-loop filtering procedure for a reconstructed picturein step S1350. The picture reconstruction procedure may be performed onthe basis of prediction samples and residual samples that are obtainedthrough the inter/intra prediction in step S1320 and the residualprocessing in step S1330 (dequantization and inverse transformation ofquantized transformation coefficients) process described in the presentdocument. A modified reconstructed picture may be generated through thein-loop filtering procedure for a reconstructed picture generatedthrough the picture reconstruction procedure. The modified reconstructedpicture may be output as a decoded picture, and may be stored in thedecoded picture buffer or the memory 250 of the decoding apparatus to beused later as a reference picture in the inter prediction procedure whena picture is decoded. In some cases, the in-loop filtering procedure maybe omitted. In this case, the reconstructed picture may be output as adecoded picture, and may be stored in the decoded picture buffer or thememory 250 of the decoding apparatus to be used later as a referencepicture in the inter prediction procedure when a picture is decoded. Asdescribed above, the in-loop filtering procedure in step S1350 mayinclude a deblocking filtering procedure, a sample adaptive offset (SAO)procedure, an adaptive loop filter (ALF) procedure, and/or a bi-lateralfilter procedure. Some or all thereof may be omitted. In addition, oneor some of the deblocking filtering procedure, the sample adaptiveoffset (SAO) procedure, the adaptive loop filter (ALF) procedure, andthe bi-lateral filter procedure may be sequentially applied, or all ofthem may be sequentially applied. For example, the deblocking filteringprocedure may be applied to a reconstructed picture, and then the SAOprocedure may be performed. Alternatively, for example, the deblockingfiltering procedure may be applied to a reconstructed picture, and thenthe ALF procedure may be performed. This may be performed in the samemanner as in the encoding apparatus.

FIG. 14 shows an example of a schematic picture encoding procedure towhich an embodiment(s) of the present document is applicable. In FIG.14, step S1410 may be performed by the prediction unit including theintra prediction unit 185 or the inter prediction unit 180 of theencoding apparatus described above with reference to FIG. 2. Step S1420may be performed by the residual processor including the transformer 120and/or the quantizer 130. Step S1430 may be performed by the entropyencoder 190. Step S1410 may include the inter/intra prediction proceduredescribed in the present document. Step S1420 may include the residualprocessing procedure described in the present document. Step S1430 mayinclude the information encoding procedure described in the presentdocument.

Referring to FIG. 14, the picture encoding procedure may schematicallyinclude, as described above with reference to FIG. 2, a procedure forencoding information (e.g., prediction information, residualinformation, and partitioning information) for picture reconstructionand outputting the information in the form of a bitstream, a procedurefor generating a reconstructed picture for a current picture, and aprocedure (optional) for applying in-loop filtering to the reconstructedpicture. The encoding apparatus may derive (modified) residual samplesfrom quantized transformation coefficients through the dequantizer 140and the inverse transformer 150, and may generate a reconstructedpicture on the basis of the prediction samples that are an output instep S1410, and the (modified) residual samples. The generatedreconstructed picture may be the same as the reconstructed picturegenerated by the decoding apparatus described above. The in-loopfiltering procedure may be performed on the reconstructed picture togenerate a modified reconstructed picture. The modified reconstructedpicture may be stored in the decoded picture buffer or the memory 170.Similarly to the case in the decoding apparatus, the modifiedreconstructed picture may be used later as a reference picture in theinter prediction procedure when a picture is encoded. As describedabove, in some cases, some or all of the in-loop filtering procedure maybe omitted. When the in-loop filtering procedure is performed, (in-loop)filtering-related information (parameter) may be encoded by the entropyencoder 190 and output in the form of a bitstream. The decodingapparatus may perform the in-loop filtering procedure on the basis ofthe filtering-related information in the same manner as the encodingapparatus.

Through this in-loop filtering procedure, noises, such as a blockingartifact and a ringing artifact, generating during image/video codingmay be reduced, and subjective/objective visual quality may be improved.In addition, both the encoding apparatus and the decoding apparatusperform the in-loop filtering procedure, so that the encoding apparatusand the decoding apparatus may derive the same prediction result, thereliability of picture coding may be increased, and the amount of datato be transmitted for picture coding may be reduced.

As described above, the picture reconstruction procedure may beperformed in the decoding apparatus as well as the encoding apparatus. Areconstructed block may be generated on the basis of intraprediction/inter prediction based on each block basis, and areconstructed picture including reconstructed blocks may be generated.When a current picture/slice/tile group is an I picture/slice/tilegroup, the blocks included in the current picture/slice/tile group maybe reconstructed on the basis of only intra prediction. In the meantime,when a current picture/slice/tile group is a P or B picture/slice/tilegroup, the blocks included in the current picture/slice/tile group maybe reconstructed on the basis of intra prediction or inter prediction.In this case, inter prediction may be applied to some blocks in thecurrent picture/slice/tile group, and intra prediction may be applied tosome remaining blocks. A color component of a picture may include a lumacomponent and a chroma component. Unless explicitly limited in thepresent document, the method and the embodiments proposed in the presentdocument may be applied to the luma component and the chroma component.

Example of Coding Hierarchy and Structure

A coded video/image according to the present document may be processedaccording to, for example, a coding hierarchy and structure to bedescribed later.

FIG. 15 is a view showing a hierarchical structure for a coded image. Acoded image may be divided into a video coding layer (VCL, video codinghierarchy) that handles decoding processing of an image and itself, asubsystem for transmitting and storing encoded information, and anetwork abstraction layer (NAL, network abstraction hierarchy) that ispresent between the VCL and the subsystem, and is in charge of a networkadaptation function.

In the VCL, VCL data including compressed image data (slice data) may begenerated. Alternatively, a parameter set including information, such asa picture parameter set (PPS), a sequence parameter set (SPS), and avideo parameter set (VPS), or a supplemental enhancement information(SEI) message additionally required for an image decoding process may begenerated.

In the NAL, header information (NAL unit header) is added to a raw bytesequence payload (RBSP) generated in the VCL so that an NAL unit may begenerated. Herein, the RBSP refers to slice data, a parameter set, andan SEI message generated in the VCL. The NAL unit header may include NALunit type information that is specified according to RB SP data includedin the NAL unit.

As shown in the figure, the NAL unit may be divided into a VCL NAL unitand a non-VCL NAL unit depending on the RBSP generated in the VCL. TheVCL NAL unit may refer to an NAL unit that includes information (slicedata) on an image. The non-VCL NAL unit may refer to an NAL unit thatincludes information (a parameter set or an SEI message) required fordecoding an image.

With header information attached according to a data standard of thesubsystem, the VCL NAL unit and the non-VCL NAL unit may be transmittedover a network. For example, an NAL unit may be transformed in the formof data of a predetermined standard, such as H.266/VVC file format, areal-time transport protocol (RTP), a transport stream (TS), and theresulting data may be transmitted over various networks.

As described above, regarding an NAL unit, an NAL unit type may bespecified according to an RBSP data structure included in the NAL unit,and information on the NAL unit type may be stored in an NAL unit headerand signaled.

For example, depending on whether an NAL unit includes information(slice data) on an image, rough classification into a VCL NAL unit typeand a non-VCL NAL unit type is made. The VCL NAL unit type may beclassified according to a characteristic and a type of a pictureincluded in the VCL NAL unit, and the non-VCL NAL unit type may beclassified according to the type of a parameter set.

NAL unit types specified according to types of parameter sets whichnon-VCL NAL unit types include are listed below as an example.

-   -   APS (Adaptation Parameter Set) NAL unit: a type of an NAL unit        including an APS    -   DPS (Decoding Parameter Set) NAL unit: a type of an NAL unit        including a DPS    -   VPS (Video Parameter Set) NAL unit: a type of an NAL unit        including a VPS    -   SPS (Sequence Parameter Set) NAL unit: a type of an NAL unit        including an SPS    -   PPS (Picture Parameter Set) NAL unit: a type of an NAL unit        including a PPS

The above-described NAL unit types have syntax information for an NALunit type, and the syntax information may be stored in the NAL unitheader and signaled. For example, the syntax information may benal_unit_type, and NAL unit types may be specified by nal_unit_typevalues.

The slice header (slice header syntax) may include information/aparameter applicable to the slice in common. The APS (APS syntax) or thePPS (PPS syntax) may include information/a parameter applicable to oneor more slices or pictures in common. The SPS (SPS syntax) may includeinformation/a parameter applicable to one or more sequences in common.The VPS (VPS syntax) may include information/a parameter applicable tomultiple layers in common. The DPS (DPS syntax) may includeinformation/a parameter applicable to the entire video in common. TheDPS may include information/a parameter related to concatenation ofcoded video sequences (CVSs). In the present document, a high levelsyntax (HLS) may include at least one selected from the group of the APSsyntax, the PPS syntax, the SPS syntax, the VPS syntax, the DPS syntax,and the slice header syntax.

In the present document, image/video information encoded by the encodingapparatus and signaled in the form of a bitstream to the decodingapparatus may include partitioning-related information, intra/interprediction information, residual information, and in-loop filteringinformation within a picture as well as information included in theslice header, information included in the APS, information included inthe PPS, information included in the SPS, and/or information included inthe VPS.

Overview of Intra Prediction

Hereinafter, intra prediction performed by the encoding apparatus andthe decoding apparatus described above will be described in detail.Intra prediction may refer to prediction that generates predictionsamples for a current block on the basis of reference samples in apicture (hereinafter, a current picture) to which the current blockbelongs.

This will be described with reference to FIG. 16. When intra predictionis applied to a current block 1601, neighboring reference samples to beused for intra prediction of the current block 1601 may be derived. Theneighboring reference samples of the current block may include: a totalof 2×nH samples including samples 1611 adjacent to a left boundary ofthe current block having a size of nW×nH and samples 1612 neighboringthe bottom-left; a total of 2×nW samples including samples 1621 adjacentto the top boundary of the current block and samples 1622 neighboringthe top-right; and one sample 1631 neighboring the top-left of thecurrent block. Alternatively, the neighboring reference samples of thecurrent block may include a plurality of columns of top neighboringsamples and a plurality of rows of left neighboring samples.

In addition, the neighboring reference samples of the current block mayinclude: a total of nH samples 1641 adjacent to the right boundary ofthe current block having a size of nW×nH; a total of nW samples 1651adjacent to the bottom boundary of the current block; and one sample1642 neighboring the bottom-right of the current block.

However, some of the neighboring reference samples of the current blockhave not yet been decoded or may be unavailable. In this case, thedecoding apparatus may construct neighboring reference samples to beused for prediction, by substituting unavailable samples with availablesamples. Alternatively, neighboring reference samples to be used forprediction may be constructed through interpolation of availablesamples.

When the neighboring reference samples are derived, (i) a predictionsample may be derived on the basis of an average or interpolation of theneighboring reference samples of the current block, or (ii) theprediction sample may be derived on the basis of the reference samplepresent in a particular (prediction) direction with respect to theprediction sample, among the neighboring reference samples of thecurrent block. The case of (i) may be referred to as a non-directionalmode or a non-angular mode, and the case of (ii) may be referred to as adirectional mode or an angular mode. In addition, the prediction samplemay be generated through interpolation with the second neighboringsample and the first neighboring sample that are located in the oppositedirection of the prediction direction of the intra prediction mode ofthe current block on the basis of the prediction sample of the currentblock, among the neighboring reference samples. The above-described casemay be referred to as a linear interpolation intra prediction (LIP). Inaddition, chroma prediction samples may be generated on the basis ofluma samples by using a linear model. This case may be referred to as anLM mode. In addition, a temporary prediction sample of the current blockmay be derived on the basis of filtered neighboring reference samples,and the prediction sample of the current block may be derived byweighted-summing the temporary prediction sample and at least onereference sample derived according to the intra prediction mode amongthe existing neighboring reference samples, namely, the unfilteredneighboring reference samples. The above-described case may be referredto as Position dependent intra prediction (PDPC). In addition, areference sample line having the highest prediction accuracy may beselected among multiple neighboring reference sample lines of thecurrent block to derive a prediction sample by using a reference samplelocated in a prediction direction in the corresponding line. At thistime, intra prediction encoding may be performed by indicating(signaling) the used reference sample line to the decoding apparatus.The above-described case may be referred to as multi-reference line(MRL) intra prediction or MRL based intra prediction. In addition, thecurrent block may be divided into vertical or horizontal sub-partitionsto perform intra prediction based on the same intra prediction mode, andneighboring reference samples may be derived on a per sub-partitionbasis and used. That is, in this case, the intra prediction mode for thecurrent block is equally applied to the sub-partitions, and aneighboring reference sample is derived on a per sub-partition basis andused, thereby increasing intra prediction performance in some cases.This prediction method may be referred to as intra sub-partitions (ISP)or ISP based intra prediction. These intra prediction methods may bereferred to as intra prediction types, being distinguished from intraprediction modes (e.g., a DC mode, a planar mode, and a directionalmode). The intra prediction types may be referred to as various termssuch as intra prediction schemes or additional intra prediction modes.For example, the intra prediction types (or additional intra predictionmodes) may include at least one selected from a group of LIP, PDPC, MRL,and ISP that are described above. A general intra prediction methodexcluding the particular intra prediction types, such as LIP, PDPC, MRL,and ISP, may be referred to as a normal intra prediction type. Thenormal intra prediction type may refer to a case in which the particularintra prediction types are not applied, and prediction may be performedon the basis of the above-described intra prediction modes. In themeantime, when necessary, post-filtering may be performed on the derivedprediction sample.

Specifically, an intra prediction procedure may include an intraprediction mode/type determination step, a neighboring reference samplederivation step, and an intra prediction mode/type based predictionsample derivation step. In addition, when necessary, a post-filteringstep may be performed on the derived prediction sample.

In the meantime, in addition to the above-described intra predictiontypes, affine linear weighted intra prediction (ALWIP) may be used. TheALWIP may be referred to as linear weighted intra prediction (LWIP), ormatrix weighted intra prediction or matrix based intra prediction (MIP).When the MIP is applied to a current block, prediction samples for thecurrent block may be derived by i) using neighboring reference samplessubjected to an averaging procedure, ii) performing amatrix-vector-multiplication procedure, and further performing iii) ahorizontal/vertical interpolation procedure when necessary. The intraprediction modes used for the MIP may be different from the intraprediction modes used in LIP, PDPC, MRL, ISP intra prediction, or innormal intra prediction. The intra prediction modes for the MIP may bereferred to as MIP intra prediction modes, MIP prediction modes, or MIPmodes. For example, different matrices and offsets used in thematrix-vector-multiplication may be set according to the intraprediction modes for MIP. Herein, a matrix may be referred to as a (MIP)weighted matrix, and an offset may be referred to as an (MIP) offsetvector or (MIP) bias vector. A detailed MIP method will be describedlater.

A block reconstruction procedure based on intra prediction and the intraprediction unit in the encoding apparatus may schematically include, forexample, the following described below. Step S1710 may be performed bythe intra prediction unit 185 of the encoding apparatus. Step S1720 maybe performed by the residual processor that includes at least oneselected from a group of the subtractor 115, the transformer 120, thequantizer 130, the dequantizer 140, and the inverse transformer 150 ofthe encoding apparatus. Specifically, step S1720 may be performed by thesubtractor 115 of the encoding apparatus. In step S1730, predictioninformation may be derived by the intra prediction unit 185, and may beencoded by the entropy encoder 190. In step S1730, residual informationmay be derived by the residual processor, and may be encoded by theentropy encoder 190. The residual information is information on theresidual samples. The residual information may include information onquantized transformation coefficients for the residual samples. Asdescribed above, the residual samples may be derived as transformationcoefficients through the transformer 120 of the encoding apparatus, andthe transformation coefficients may be derived as quantizedtransformation coefficients through the quantizer 130. Information onthe quantized transformation coefficients may be encoded by the entropyencoder 190 through a residual coding procedure.

The encoding apparatus may perform intra prediction on a current blockin step S1710. The encoding apparatus derives an intra predictionmode/type for the current block, derives neighboring reference samplesof the current block, and generates prediction samples in the currentblock on the basis of the intra prediction mode/type and the neighboringreference samples. Herein, the procedures of determination of the intraprediction mode/type, derivation of the neighboring reference samples,and generation of the prediction samples may be performedsimultaneously, or any one procedure may be performed before the otherprocedures. For example, although not shown, the intra prediction unit185 of the encoding apparatus may include an intra prediction mode/typedetermination unit, a reference sample derivation unit, and a predictionsample derivation unit. The intra prediction mode/type determinationunit may determine an intra prediction mode/type for the current block,the reference sample derivation unit may derive neighboring referencesamples of the current block, and the prediction sample derivation unitmay derive prediction samples of the current block. In the meantime,when a prediction sample filtering procedure, which will be describedlater, is performed, the intra prediction unit 185 may further include aprediction sample filter. The encoding apparatus may determine amode/type applied to the current block among a plurality of intraprediction modes/types. The encoding apparatus may compare RD costs forthe intra prediction modes/types and determine the optimum intraprediction mode/type for the current block.

In the meantime, the encoding apparatus may perform a prediction samplefiltering procedure. Prediction sample filtering may be referred to aspost-filtering. By the prediction sample filtering procedure, some orall of the prediction samples may be filtered. In some cases, theprediction sample filtering procedure may be omitted.

The encoding apparatus may generate residual samples for the currentblock on the basis of (filtered) prediction samples in step S1720. Theencoding apparatus may compare the prediction samples in the originalsamples of the current block on the basis of a phase, and may derive theresidual samples.

The encoding apparatus may encode image information includinginformation (prediction information) on the intra prediction andresidual information on the residual samples in step S1730. Theprediction information may include the intra prediction mode informationand the intra prediction type information. The encoding apparatus mayoutput encoded image information in the form of a bitstream. The outputbitstream may be transmitted to the decoding apparatus through a storagemedium or a network.

The residual information may include residual coding syntax, which willbe described later. The encoding apparatus may derive quantizedtransformation coefficients by transforming/quantizing the residualsamples. The residual information may include information on thequantized transformation coefficients.

In the meantime, as described above, the encoding apparatus may generatea reconstructed picture (including reconstructed samples and areconstructed block). To this end, the encoding apparatus may performdequantization/inverse transformation on the quantized transformationcoefficients and derive (modified) residual samples. The reason forperforming dequantization/inverse transformation aftertransformation/quantization of the residual samples is to deriveresidual samples that are the same as the residual samples derived bythe decoding apparatus as described above. The encoding apparatus maygenerate a reconstructed block including reconstructed samples for thecurrent block, on the basis of the prediction samples and the (modified)residual samples. On the basis of the reconstructed block, areconstructed picture for the current picture may be generated. Asdescribed above, an in-loop filtering procedure may be further appliedto the reconstructed picture.

A video/image decoding procedure based on intra prediction and the intraprediction unit in the decoding apparatus may schematically include, forexample, the following described below. The decoding apparatus mayperform the operation corresponding to the operation performed by theencoding apparatus.

Steps S1810 to S1830 may be performed by the intra prediction unit 265of the decoding apparatus. Prediction information in step S1810 andresidual information in step S1840 may be obtained from a bitstream bythe entropy decoder 210 of the decoding apparatus. A residual processorincluding the dequantizer 220 or the inverse transformer 230 of thedecoding apparatus or both may derive residual samples for the currentblock on the basis of the residual information. Specifically, thedequantizer 220 of the residual processor may perform dequantization onthe basis of quantized transformation coefficients derived on the basisof the residual information, and may derive transformation coefficients.The inverse transformer 230 of the residual processor may performinverse transformation on the transformation coefficients and may deriveresidual samples for the current block. Step S1850 may be performed bythe adder 235 or the reconstructor of the decoding apparatus.

Specifically, the decoding apparatus may derive an intra predictionmode/type for a current block on the basis of received predictioninformation (intra prediction mode/type information) in step S1810. Thedecoding apparatus may derive neighboring reference samples of thecurrent block in step S1820. The decoding apparatus may generateprediction samples in the current block on the basis of the intraprediction mode/type and the neighboring reference samples in stepS1830. In this case, the decoding apparatus may perform a predictionsample filtering procedure. Prediction sample filtering may be referredto as post-filtering. By the prediction sample filtering procedure, someor all of the prediction samples may be filtered. In some cases, theprediction sample filtering procedure may be omitted.

The decoding apparatus may generate residual samples for the currentblock on the basis of received residual information. The decodingapparatus may generate reconstructed samples for the current block onthe basis of the prediction samples and the residual samples, and mayderive a reconstructed block including the reconstructed samples in stepS1840. On the basis of the reconstructed block, a reconstructed picturefor the current picture may be generated. As described above, an in-loopfiltering procedure may be further applied to the reconstructed picture.

Herein, although not shown, the intra prediction unit 265 of thedecoding apparatus may include an intra prediction mode/typedetermination unit, a reference sample derivation unit, and a predictionsample derivation unit. The intra prediction mode/type determinationunit may determine an intra prediction mode/type for the current blockon the basis of the intra prediction mode/type information obtained fromthe entropy decoder 210. The reference sample derivation unit may deriveneighboring reference samples of the current block. The predictionsample derivation unit may derive prediction samples of the currentblock. In the meantime, when the above-described prediction samplefiltering procedure is performed, the intra prediction unit 265 mayfurther include a prediction sample filter.

The intra prediction mode information may include, for example, flaginformation (e.g., intra_luma_mpm_flag) indicating whether a mostprobable mode (MPM) is applied to the current block or a remaining modeis applied. When the MPM is applied to the current block, the predictionmode information may further include index information (e.g.,intra_luma_mpm_idx) indicating one of the intra prediction modecandidates (MPM candidates). The intra prediction mode candidates (MPMcandidates) may be constructed as a MPM candidate list or an MPM list.In addition, when the MPM is not applied to the current block, the intraprediction mode information may further include remaining modeinformation (e.g., intra_luma_mpm_remainder) indicating one of theremaining intra prediction modes except the intra prediction modecandidates (MPM candidates). The decoding apparatus may determine anintra prediction mode of the current block on the basis of the intraprediction mode information. A separate MPM list may be constructed forthe above-described MIP.

In addition, the intra prediction type information may be realized invarious forms. For example, the intra prediction type information mayinclude intra prediction type index information indicating one of theintra prediction types. As another example, the intra prediction typeinformation may include at least one of reference sample lineinformation (e.g., intra_luma_ref_idx) indicating whether the MRL isapplied to the current block and which reference sample line is usedwhen the MRL is applied to the current block, ISP flag information(e.g., intra_subpartitions_mode_flag) indicating whether the ISP isapplied to the current block, ISP type information (e.g.,intra_subpartitions_split_flag) indicating a partition type ofsub-partitions when the ISP is applied, flag information indicatingwhether PDCP is applied, or flag information indicating whether LIP isapplied. In addition, the intra prediction type information may includean MIP flag indicating whether MIP is applied to the current block.

The intra prediction mode information and/or the intra prediction typeinformation may be encoded/decoded through a coding method described inthe present document. For example, the intra prediction mode informationand/or the intra prediction type information may be encoded/decodedthrough entropy coding (e.g., CABAC, CAVLC) on the basis of a truncated(rice) binary code.

Overview of Adaptive Color Transform (ACT)

Adaptive color transform (ACT) is a color space transformation(conversion) technology for removing unnecessary overlap between colorcomponents, and has been used in an HEVC screen content extensionversion. This may also be applied to VVC.

In the HEVC screen content extension (HEVC SCC extension), ACT has beenused to adaptively transform a prediction residual from an existingcolor space to a YCgCo color space. One of the two color space may beoptionally selected by signaling one ACT flag for each transformationbasis.

For example, a first value (e.g., 1) of the flag may indicate that aresidual of the transformation basis is encoded in the original colorspace. A second value (e.g., 1) of the flag may indicate that a residualof the transformation basis is encoded in the YCgCo color space.

FIG. 19 is a view showing an embodiment of a decoding process to whichACT is applied. In an embodiment of FIG. 19, motion compensatedprediction may correspond to inter prediction in the present disclosure.

As shown in FIG. 19, a reconstructed picture (or a reconstructed block,a reconstructed sample array, a reconstructed sample(s), a reconstructedsignal) may be generated on the basis of a prediction output value and aresidual output value. Herein, the residual output value may be aninverse transformation output value. Herein, inverse transformation maybe normal inverse transformation. Herein, the normal inversetransformation may be MTS -based inverse transformation or inverse lowfrequency non-separable transform (LFNST).

Herein, the prediction output value may be a prediction block, aprediction sample array, a prediction sample(s) or a prediction signal.The residual output value may be a residual block, a residual samplearray, a residual sample(s), or a residual signal.

For example, in terms of the encoding apparatus, an ACT process may beperformed on residual samples derived on the basis of predictionsamples. In addition, an output value of the ACT process may be providedas an input of a normal transformation process. Herein, the normaltransformation process may be MTS -based transformation or LFNST.

Information (parameter) on (inverse) ACT may be generated and encoded bythe encoding apparatus, and may be transmitted to the decoding apparatusin the form of a bitstream.

The decoding apparatus may obtain, parse, and decode (inverse)ACT-related information (parameter), and may perform inverse ACT on thebasis of the (inverse) ACT-related information (parameter).

On the basis of the inverse ACT, (modified) residual samples (orresidual block) may be derived. For example, (transformation)coefficients may be derived by applying dequantization to quantized(transformation) coefficients. In addition, residual samples may bederived by performing inverse transformation on (transformation)coefficients. In addition, (modified) residual samples may be obtainedby applying inverse ACT to residual samples. The information (parameter)on (inverse) ACT will be described in detail later.

In an embodiment, a core transformation function used in HEVC may beused as a core transformation function (transformation kernel) for colorspace transformation. For example, matrices for forward transformationand backward transformation as shown in the equations below may be used.

$\begin{matrix}{\begin{bmatrix}C_{0}^{\prime} \\C_{1}^{\prime} \\C_{2}^{\prime}\end{bmatrix} = {{\begin{bmatrix}2 & 1 & 1 \\2 & {- 1} & {- 1} \\0 & {- 2} & 2\end{bmatrix}\begin{bmatrix}C_{0} \\C_{1} \\C_{2}\end{bmatrix}}/4}} & \lbrack {{Equation}1} \rbrack\end{matrix}$ $\begin{matrix}{\begin{bmatrix}C_{0} \\C_{1} \\C_{2}\end{bmatrix} = {\begin{bmatrix}1 & 1 & 0 \\1 & {- 1} & {- 1} \\1 & {- 1} & 1\end{bmatrix}\begin{bmatrix}C_{0}^{\prime} \\C_{1}^{\prime} \\C_{2}^{\prime}\end{bmatrix}}} & \lbrack {{Equation}2} \rbrack\end{matrix}$

Herein, C0, C1, and C2 may correspond to G, B, and R. Herein, G denotesa green color component, B denotes a blue color component, and R denotesa red color component. In addition, C0′, C1′ , and C2′ may correspond toY, Cg, and Co. Herein, Y denotes a luma, Cg denotes a green chroma, andCo denotes an orange chroma component.

In addition, in order to compensate for a dynamic range change of aresidual before and after color transformation, QP adjustment may beapplied to a transformation residual by (−5, −5, −3). Details of QPadjustment will be described later.

In the meantime, in the encoding and decoding processes according to anembodiment, when ACT is applicable, the following limitations may beapplied.

-   -   In the case of dual tree encoding/decoding, ACT is deactivated.        For example, ACT may be applied only to single-tree        encoding/decoding.    -   When ISP encoding and decoding are applied, ACT may be        deactivated.    -   For the chroma block to which BDPCM is applied, ACT may be        deactivated. Only for the luma block to which BDPCM is applied,        ACT may be activated.    -   When application of ACT is possible, the CCLM may be        deactivated.

FIG. 20 is a view showing an embodiment of a sequence parameter setsyntax table in which a syntax element related to ACT is signaled.

FIGS. 21 to 27 are views successively showing an embodiment of a syntaxtable of an encoding basis in which a syntax element related to ACT issignaled.

As shown in FIG. 20, as an ACT activation flag indicating whether ACT isactivated in the decoding process, sps_act_enabled_flag 2010 may beused.

A first value (e.g., 0) of the sps_act_enabled_flag may indicate thatACT is not used and a flag cu_act_enabled_flag 2110, 2710 indicatingwhether ACT is applied in the encoding basis are not provided in thesyntax for the encoding basis.

A second value (e.g., 1) of the sps_act_enabled_flag may indicate thatACT may be used and the cu_act_enabled_flag may be provided in thesyntax for the encoding basis.

When the sps_act_enabled_flag is not obtained in a bitstream, the valueof the sps_act_enabled_flag may be derived as the first value (e.g., 0).

In addition, as shown in FIG. 21, as ACT flags indicating whether aresidual of the current encoding basis is encoded in the YCgCo colorspace, the cu_act_enabled_flag 2110, 2710 may be used.

A first value (e.g., 0) of the cu_act_enabled_flag may indicate that aresidual of the current encoding basis is encoded in the original colorspace. A second value (e.g., 1) of the cu_act_enabled_flag may indicatethat a residual of the current encoding basis is encoded in the YCgCocolor space.

When the cu_act_enabled_flag is not provided in a bitstream, the flagmay be derived as the first value (e.g., 0). Herein, the original colorspace may be an RGB color space.

OP Derivation Method of Transformation Basis by Using ACT QP Offset

In an embodiment, in a scaling process for a transformation coefficient,a derivation process of a quantization parameter and a Qp update processmay be performed as follows. For example, the quantization parameterderivation process may be performed using the following parameters.

-   -   Luma coordinates (xCb, yCb) indicating relative coordinates of        the top left luma sample of a current encoding block with        respect to the top left luma sample of a current picture,    -   A variable cbWidth indicating the width of the current encoding        block on a per luma sample basis,    -   A variable cbHeight indicating the height of the current        encoding block on a per luma sample basis    -   A variable treeType indicating whether a single tree        (SINGLE_TREE) or a dual tree is used to partition a current        coding tree node, and indicating, when the dual tree is used,        whether the dual tree is a luma component dual tree        (DAUL_TREE_LUMA) or a chroma component dual tree        (DAUL_TREE_CHROMA)

In the present process, a luma quantization parameter Qp′Y, chromaquantization parameters Qp′Cb, Qp′Cr, and Qp′CbCr may be derived.

A variable luma location (xQg, yQg) may indicate the location of the topleft luma sample of a current quantization group corresponding to thetop left sample of a current picture. Herein, the horizontal locationxQg and the vertical location yQg may be set to be equal to the valuesof a variable CuQgTopLeftX and a variable CuQgTopLeftY, respectively.The variables CuQgTopLeftX and CuQgTopLeftY may be defined aspredetermined values in the coding tree syntax as shown in FIG. 28.

Herein, the current quantization group may be a quadrangular area withina coding tree block, and may share the same qP_(Y_PRED) value. The widthand the height thereof may be equal to the width and the height of acoding tree node in which a top left luma sample location is assigned toeach of the CuQgTopLeftX and the CuQgTopLeftY.

When the treeType is the SINGLE_TREE or the DUAL_TREE_LUMA, a lumaquantization parameter prediction value qP_(Y_PRED) may be derived as infollowing steps.

1. The variable qP_(Y_PRED) may be derived as follows.

(Condition 1) When any one of the following conditions is true, thevalue of the qP_(Y_PRED) may be set to the same value as SliceQp_(Y)(herein, SliceQp_(Y) indicates an initial value of a quantizationparameter Qp_(Y) for all slices in a picture, and this may be obtainedfrom a bitstream). Alternatively, the value of the qP_(Y_PRED) may beset to the value of the luma quantization parameter Qp_(Y) of the lastluma encoding basis of the immediately preceding quantization groupaccording to the decoding order.

-   -   (Condition 1-1) When the current quantization group is the first        quantization group in a slice    -   (Condition 1-2) When the current quantization group is the first        quantization group in a tile    -   (Condition 1-3) When the current quantization group is the first        quantization group in a CTB row in a tile and predetermined        synchronization occurs (e.g., when        entropy_coding_sync_enabled_flag has a value of 1)

2. A value of a variable qP_(Y_A) may be derived as follows.

(Condition 2) When at least one of the following conditions is true, thevalue of the qP_(Y_A) may be set to the value of the qP_(Y_PRED).Alternatively, the value of the qP_(Y_A) may be set to the value of theluma quantization parameter Qp_(Y) of the encoding basis that includes aluma encoding block covering a luma sample location (xQg−1, yQg).

-   -   (Condition 2-1) With respect to a block identified by a sample        location (xCb, yCb), a block identified by a sample location        (xQg−1, yQg) is not an available neighboring block,    -   (Condition 2-2) When a CTB including a luma encoding block        covering a luma sample location (xQg−1, yQg) is different from a        CTB including a current luma encoding block at a luma sample        location (xCb, yCb), for example, when all the following        condition are true    -   (Condition 2-2-1) The value of (xQg−1)>>CtbLog2SizeY is        different from that of (xCb)>>CtbLog2SizeY    -   (Condition 2-2-2) The value of (yQg)>>CtbLog2SizeY is different        from that of (yCb)>>CtbLog2SizeY

3. A value of a variable qP_(Y_B) may be derived as follows.

(Condition 3) When at least one of the following conditions is true, thevalue of the qP_(Y_B) may be set to the value of the qP_(Y_PRED).Alternatively, the value of the qP_(Y_B) may be set to the value of theluma quantization parameter Qp_(Y) of the encoding basis that includes aluma encoding block covering a luma sample location (xQg, yQg−1).

-   -   (Condition 3-1) With respect to a block identified by a sample        location (xCb, yCb), when a block identified by a sample        location (xQg, yQg-1) is not an available neighboring block,    -   (Condition 3-2) When a CTB including a luma encoding block        covering a luma sample location (xQg, yQg−1) is different from a        CTB including a current luma encoding block at a luma sample        location (xCb, yCb), for example, when all the following        conditions are true    -   (Condition 3-2-1) The value of (xQg)>>CtbLog2SizeY is different        from that of (xCb)>>CtbLog2SizeY    -   (Condition 3-2-2) The value of (yQg−1)>>CtbLog2SizeY is        different from that of (yCb)>>CtbLog2SizeY

4. A luma quantization parameter prediction value qP_(Y_PRED) may bederived as follows.

When all the following conditions are true, the qP_(Y_PRED) may be setto the luma quantization parameter Qp_(Y) of the encoding basis thatincludes a luma encoding block covering a luma sample location (xQg,yQg−1).

-   -   (Condition 3-1) With respect to a block identified by a sample        location (xCb, yCb), a block identified by a sample location        (xQg, yQg−1) is an available neighboring block    -   When the current quantization group is the first quantization        group in a CTB row in a tile

In the meantime, when all the conditions are not true, the qP_(Y_PRED)may be derived as shown in the equation below.

qP _(Y_PRED)=(qP _(Y_A) +qP _(Y_B)+1)>>1   [Equation 3]

The variable Qp_(Y) may be derived as shown in the equation below.

Qp_(Y)=((qP_(Y_PRED)=(CuQpDeltaVal+64+2*QpBdOffset)%(64+QpBdOffset))−QpBdOffset  [Equation 4]

Herein, the CuQpDeltaVal indicates the difference between a lumaquantization parameter for the encoding basis and a prediction valuethereof. The value thereof may be obtained from a bitstream. TheQpBdOffset indicates a luma and chroma quantization parameter rangeoffset. The QpBdOffset may be preset to a predetermined constant orobtained from a bitstream. For example, the QpBdOffset may be calculatedby multiplying a predetermined constant by a value of a syntax elementthat indicates the bit depth of a luma or chroma sample. The lumaquantization parameter Qp′_(Y) may be derived as shown in the equationbelow.

Qp′_(Y)=Qp_(Y)+QpBdOffset

When the value of a variable ChromaArrayType indicating a type of achroma array is not a first value (e.g., 0) and the treeType is theSINGLE_TREE or the DUAL_TREE_CHROMA, the following processing may beperformed.

-   -   When the value of the treeType is the DUAL_TREE_CHROMA, the        value of the variable Qp_(Y) may be set to the same value as        that of the luma quantization parameter Qp_(Y) of the luma        encoding basis covering the luma sample location (xCb+cbWidth/2,        yCb+cbHeight/2).    -   Variables qP_(Cb), qP_(Cr), and qP_(CbCr) may be derived as        shown in the equation below.

qP_(Chroma)=Clip3(−QpBdOffset, 63, Qp_(Y))

qP_(Cb)=ChromaQpTable[0][qP_(Chroma)]

qP_(Cr)=ChromaQpTable[1][qP_(Chroma)]

qP_(CbCr)=ChromaQpTable[2][qP_(Chroma)]  [Equation 6]

The chroma quantization parameters Qp′_(Cb) and Qp′_(Cr) for Cb and Crcomponents and the chroma quantization parameter Qp′_(CbCr) for jointCb-Cr coding may be derived as shown in the equation below.

Qp′_(Cb)=Clip3(−QpBdOffset, 63,qP_(Cb)+pps_cb_qp_offset+slice_cb_qp_offset+CuQpOffset_(Cb))+QpBdOffset

Qp′_(Cr)=Clip3(−QpBdOffset, 63,qP_(Cr)+pps_cr_qp_offset+slice_cr_qp_offset+CuQpOffset_(Cr))+QpBdOffset

Qp′_(CbCr)=Clip3(−QpBdOffset, 63,qP_(CbCr)+pps_joint_cbcr_qp_offset+slice_joint_cbcr_qp_offset+CuQpOffset_(CbCr))+QpBdOffset  [Equation 7]

In the equation above, the pps_cb_qp_offset and the pps_cr_qp_offset areoffsets used to derive the Qp′_(Cb) and the Qp′_(Cr), and may beobtained from a bitstream for a picture parameter set. Theslice_cb_qp_offset and the slice_cr_qp_offset are offsets used to derivethe Qp′_(Cb) and the Qp′_(Cr), and may be obtained from a bitstream fora slice header. The CuQpOffset_(Cb) and the CuQpOffset_(Cr) are offsetsused to derive the Qp′_(Cb) and the Qp′_(Cr), and may be obtained from abitstream for the transformation basis.

In addition, for example, a dequantization process for a transformationcoefficient may be performed using the following parameters.

-   -   Luma coordinates (xTbY, yTbY) referring to relative coordinates        of the top left sample of the current luma transformation block        with respect to the top left luma sample of the current picture    -   A variable nTbW indicating the width of a transformation block    -   A variable nTbH indicating the height of a transformation block    -   A variable cIdx indicating a color component of a current block

The output of the present process may be an array d of scaledtransformation coefficients. Herein, the size of the array d may be(nTbW)×(nTbH). The individual elements constituting this may beidentified as d[x][y].

To this end, a quantization parameter qP may be derived as follows. Whenthe cIdx has a value of 0, the qP may be derived as shown in theequation below.

qP=Qp′_(Y)   [Equation 8]

Alternatively, when the TuCResMode[xTbY][yTbY] has a value of 2, the qPmay be derived as shown in the equation below.

qP=Qp′_(CbCr)   [Equation 9]

Alternatively, when the cIdx has a value of 1, the qP may be derived asshown in the equation below.

qP=Qp′_(Cb)   [Equation 10]

Alternatively, when the cIdx has a value of 2, the qP may be derived asshown in the equation below.

qP=Qp′_(Cr)   [Equation 11]

Afterward, the quantization parameter qP may be updated as follows. Inaddition, variables rectNonTsFlag and bdShift may be derived as follows.For example, when transform_skip_flag[xTbY][yTbY][cIdx] has a value of 0(e.g., when transformation of a current transformation block is notskipped), the qP may be derived as shown in the equation below.

qP=qP−(cu_act_enabled_flag[xTbY][yTbY]?5:0)

rectNonTsFlag=0

bdShift=10   [Equation 12]

Alternatively, when the transform_skip_flag[xTbY][yTbY][cIdx] has avalue of 1 (e.g., transformation of the current transformation block isskipped), the qP may be derived as shown in the equation below.

qP=Max(QpPrimeTsMin, qP)−(cu_act_enabled_flag[xTbY][yTbY]?5:0)

rectNonTsFlag=(((Log2(nTbW)+Log2(nTbH))& 1)==1

bdShift=BitDepth+(rectNonTsFlag?1:0)+((Log2(nTbW)+Log2(nTbH))/2)−+pic_dep_quant_enabled_flag

Herein, the QpPrimeTsMin may indicate a minimum quantization parametervalue allowed when a transformation skip mode is applied. This may bedetermined to be a predetermined constant or may be derived from asyntax element of the bitstream related thereto.

Herein, the suffixes Y, Cb, and Cr may denote G, B, and R colorcomponents in an RGB color model, or Y, Cg, and Co color components in aYCgCo color model.

Overview of Block Difference Pulse Code Modulation (BDPCM)

The image encoding apparatus and the image decoding apparatus accordingto an embodiment may perform differential encoding of a residual signal.For example, the image encoding apparatus may encode the residual signalby subtracting a prediction signal from the residual signal of a currentblock, and the image decoding apparatus may decode the residual signalby adding the prediction signal to the residual signal of the currentblock. The image encoding apparatus and the image decoding apparatusaccording to an embodiment may perform differential encoding of theresidual signal by applying BDPCM described below.

BDPCM according to the present disclosure may be performed in aquantized residual domain. The quantized residual domain may include aquantized residual signal (or quantized residual coefficient), and, whenapplying BDPCM, transform of the quantized residual signal may beskipped. For example, when applying BDPCM, transform of the residualsignal may be skipped and quantization may be performed. Alternatively,the quantized residual domain may include quantized transformcoefficients.

In an embodiment to which BDPCM applies, the image encoding apparatusmay derive a residual block of a current block predicted in an intraprediction mode and quantize the residual block, thereby deriving aresidual block. When a differential encoding mode of the residual signalis performed with respect to the current block, the image encodingapparatus may perform differential encoding with respect to the residualblock to derive a modified residual block. In addition, the imageencoding apparatus may encode differential encoding mode informationspecifying the differential encoding mode of the residual signal and themodified residual block, thereby generating a bitstream.

More specifically, when BDPCM applies to the current block, a predictedblock (prediction block) including predicted samples of the currentblock may be generated by intra prediction. In this case, an intraprediction mode for performing intra prediction may be signaled througha bitstream and may be derived based on a prediction direction of BDPCMdescribed below. In addition, in this case, the intra prediction modemay be determined to be one of a vertical prediction direction mode or ahorizontal prediction direction mode. For example, when the predictiondirection of BDPCM is a horizontal direction, the intra prediction modemay be determined to be a horizontal prediction direction mode, and theprediction block of the current block may be generated by intraprediction of the horizontal direction. Alternatively, when theprediction direction of BDPCM is a vertical direction, the intraprediction mode may be determined to be a vertical prediction directionmode, and the prediction block of the current block may be generated byintra prediction of the vertical direction. When applying intraprediction of the horizontal direction, a value of a pixel adjacent tothe left of the current block may be determined to be a predictionsample value of samples included in a corresponding row of the currentblock. When applying intra prediction of the vertical direction, a valueof a pixel adjacent to the top of the current block may be determined tobe a prediction sample value of samples included in a correspondingcolumn of the current block. When applying BDPCM to the current block, amethod of generating the prediction block of the current block may beequally performed in an image encoding apparatus and an image decodingapparatus.

When applying BDPCM to the current block, the image encoding apparatusmay generate a residual block including residual samples of the currentblock, by subtracting the prediction sample from the current block. Theimage encoding apparatus may quantize the residual block and then encodea difference (or delta) between a quantized residual sample and apredictor of the quantized residual sample. The image decoding apparatusmay generate the quantized residual block of the current block, byobtaining the quantized residual sample of the current block based onthe predictor and the difference reconstructed from a bitstream.Thereafter, the image decoding apparatus may dequantize the quantizedresidual block and then add it to the prediction block, therebyreconstructing the current block.

FIG. 29 is a view illustrating a method of encoding a residual sample ofBDPCM according to the present disclosure. The residual block of FIG. 29may be generated by subtracting a prediction block from a current blockin an image encoding apparatus. The quantized residual block of FIG. 29may be generated by quantizing the residual block by the image encodingapparatus. In FIG. 29, r_(i, j) specifies a value of a residual sampleof the (i, j) coordinates in a current block. When the size of thecurrent block is M×N, a value i may be from 0 to M−1, inclusive. Inaddition, a value j may be from 0 to N−1, inclusive. For example, aresidual may refer to a difference between an original block and aprediction block. For example, r_(i, j) may be derived by subtractingthe value of the prediction sample from the value of an original sampleof the (i, j) coordinates in the current block. For example, r_(i, j)may be a prediction residual after horizontal intra prediction orvertical intra prediction is performed using a sample that is notfiltered from a top or left boundary sample. In the horizontal intraprediction, a value of a left neighboring pixel is copied along a linecrossing a prediction block. In the vertical intra prediction, a topneighboring line is copied to an individual line of a prediction block.

In FIG. 29, Q(r_(i, j)) refer to a value of a quantized residual sampleof coordinates (i, j) in the current block. For example, Q(r_(i, j)) mayrefer to a quantized value of r_(i, j).

Prediction of BDPCM is performed on the quantized residual samples ofFIG. 29, and a modified quantized residual block R′ having a M×N sizeincluding modified quantized residual samples r′ may be generated.

When a prediction direction of BDPCM is a horizontal direction, a valuer′_(i, j) of a modified quantized residual sample of coordinates (i, j)in the current block may be calculated as shown in the equation below.

$\begin{matrix}{r_{i,j}^{\prime} = \{ \begin{matrix}{{Q( r_{i,j} )},} & {{i = 0},} & {0 \leq j \leq ( {N - 1} )} \\{{{Q( r_{i,j} )} - {Q( r_{{({i - 1})},j} )}},} & {{1 \leq i \leq ( {M - 1} )},} & {0 \leq j \leq ( {N - 1} )}\end{matrix} } & \lbrack {{Equation}14} \rbrack\end{matrix}$

As shown in Equation 14, when the prediction direction of BDPCM is ahorizontal direction, a value Q(r_(0, j)) of a quantized residual sampleis assigned as it is to a value r′_(0, j) of coordinates (0, j). A valuer′_(i, j) of other coordinates (i, j) may be derived as a differencevalue between a value Q(r_(i, j)) of a quantized residual sample ofcoordinates (i, j) and a value Q(r_(i−1, j)) of a quantized residualsample of coordinates (i−1, j). That is, instead of encoding a valueQ(r_(i, j)) of a quantized residual sample of coordinates (i, j), adifference value calculated by using a value Q(r_(i−1, j)) of aquantized residual sample of coordinates (i−1, j) as a prediction valueis derived as a modified quantized residual sample value r′_(i, j), andthen the value r′_(i, j) is encoded.

When a prediction direction of BDPCM is a vertical direction, a value(r′_(i, j)) of a modified quantized residual sample of coordinates (i,j) in the current block may be calculated as shown in the equationbelow.

$\begin{matrix}{r_{i,j}^{\prime} = \{ \begin{matrix}{{Q( r_{i,j} )},} & {{0 \leq i \leq ( {M - 1} )},} & {j = 0} \\{{{Q( r_{i,j} )} - {Q( r_{i,{({j - 1})}} )}},} & {{0 \leq i \leq ( {M - 1} )},} & {1 \leq j \leq ( {N - 1} )}\end{matrix} } & \lbrack {{Equation}15} \rbrack\end{matrix}$

As shown in Equation 15, when the prediction direction of BDPCM is avertical direction, a value Q(r_(i, 0)) of a quantized residual sampleis assigned as it is to a value r′_(i, 0) of coordinates (i, 0). A valuer′_(i, j) of other coordinates (i, j) may be derived as a differencevalue between a value Q(r_(i, j)) of a quantized residual sample ofcoordinates (i, j) and a value Q(r_(i, j−1)) of a quantized residualsample of coordinates (i, j−1). That is, instead of encoding a valueQ(r_(i, j)) of a quantized residual sample of coordinates (i, j), adifference value calculated by using a value Q(r_(i, j−1)) of aquantized residual sample of coordinates (i, j−1) as a prediction valueis derived as a modified quantized residual sample value r′_(i, j), andthen the value r′_(i, j) is encoded.

As described above, the process of modifying a current quantizedresidual sample value by using a nearby quantized residual sample valueas a prediction value may be called BDPCM prediction.

Finally, the image encoding apparatus may encode a modified quantizedresidual block including the modified quantized residual samples and maytransmit the resulting block to the image decoding apparatus. Herein, asdescribed above, transformation of the modified quantized residual blockis not performed.

FIG. 30 is a view showing modified quantized residual blocks generatedby performing BDPCM of the present disclosure.

In FIG. 30, horizontal BDPCM shows a modified quantized residual blockgenerated according to Equation 14 when the prediction direction ofBDPCM is a horizontal direction. In addition, vertical BDPCM shows amodified quantized residual block generated according to Equation 15when the prediction direction of BDPCM is a vertical direction.

FIG. 31 is a flowchart showing a procedure for encoding a current blockby applying BDPCM in an image encoding apparatus.

First, when a current block that is an encoding target block is input instep S3110, prediction may be performed on the current block to generatea prediction block in step S3120. The prediction block in step S3120 maybe an intra-predicted block, and an intra prediction mode may bedetermined as described above. On the basis of the prediction blockgenerated in step S3120, a residual block of the current block may begenerated in step S3130. For example, the image encoding apparatus maygenerate a residual block (values of residual samples) by subtracting aprediction block (values of predicted samples) from the current block(values of original samples). For example, by performing step S3130, aresidual block of FIG. 29 may be generated. On the residual blockgenerated in step S3130, quantization may be performed in step S3140 togenerate a quantized residual block, and BDPCM prediction may beperformed on the quantized residual block in step S3150. The quantizedresidual block generated as a result of performing step S3140 may be thequantized residual block of FIG. 29. As a result of BDPCM prediction instep S3150, a modified quantized residual block of FIG. 30 may begenerated according to a prediction direction. Since BDPCM prediction instep S3150 has been described with reference to FIGS. 29 to 30, adetailed description thereof will be omitted. Afterward, the imageencoding apparatus may encode the modified quantized residual block instep S3160 to generate a bitstream. Herein, transformation of themodified quantized residual block may be skipped.

The BDPCM operation in the image encoding apparatus described withreference to FIGS. 29 to 31 may be performed in reverse by the imagedecoding apparatus.

FIG. 32 is a flowchart showing a procedure for reconstructing a currentblock by applying BDPCM in the image decoding apparatus.

The image decoding apparatus may obtain information (image information)required for reconstructing the current block from a bitstream in stepS3210. The information required for reconstructing the current block mayinclude information (prediction information) on prediction of thecurrent block, and information (residual information) on a residual ofthe current block. The image decoding apparatus may perform predictionon the current block on the basis of information on the current block,and may generate a prediction block in step S3220. Prediction on thecurrent block may be intra prediction, and a detailed descriptionthereof is the same as that described above with reference to FIG. 31.In FIG. 32, it is shown that step S3220 of generating the predictionblock for the current block is performed before steps S3230 to S3250 ofgenerating a residual block of the current block. However, no limitationthereto is imposed. After a residual block of the current block isgenerated, a prediction block of the current block may be generated.Alternatively, a residual block of the current block and a predictionblock of the current block may be generated simultaneously.

The image decoding apparatus may generate a residual block of thecurrent block in step S3230 by parsing residual information of thecurrent block from the bitstream. The residual block generated in stepS3230 may be the modified quantized residual block shown in FIG. 30.

The image decoding apparatus may generate the quantized residual blockof FIG. 29 by performing BDPCM prediction in step S3240 on the modifiedquantized residual block of FIG. 30. BDPCM prediction in step S3240 is aprocedure for generating the quantized residual block of FIG. 29 fromthe modified quantized residual block of FIG. 30, which corresponds tothe reverse process of step S3150 performed by the image encodingapparatus. For example, when differential encoding mode information(e.g., bdpcm_flag) obtained from a bitstream indicates a differentialencoding mode in which differential encoding of a residual coefficientis performed as BDPCM is applied, the image decoding apparatus performsdifferential encoding on a residual block to derive a modified residualblock. Using a residual coefficient to be modified and a predictionresidual coefficient, the image decoding apparatus may modify at leastone residual coefficient to be modified among residual coefficients in aresidual block. The prediction residual coefficient may be determined onthe basis of the prediction direction indicated by differential encodingdirection information (e.g., bdpcm_dir_flag) obtained from thebitstream. The differential encoding direction information may indicateeither a vertical direction or a horizontal direction. The imagedecoding apparatus may assign a value obtained by adding the residualcoefficient to be modified and the prediction residual coefficient, to alocation of the residual coefficient to be modified. Herein, theprediction residual coefficient may be a coefficient that is immediatelybefore and adjacent to the residual coefficient to be modified, in termsof the order according to the prediction direction.

Hereinafter, BDPCM prediction in step S3240 performed by the imagedecoding apparatus will be described in more detail below. The decodingapparatus may calculate a quantized residual sample Q(r_(i, j)) byperforming the calculation performed by the encoding apparatus inreverse. For example, when the prediction direction of BDPCM is ahorizontal direction, the image decoding apparatus may generate aquantized residual block from a modified quantized residual block byusing Equation 16.

Q(r _(i,j))=Σ_(k=0) ^(i) r′ _(k,j), 0≤i≤(M−1), 0≤j≤(N−1)   [Equation 16]

As defined in Equation 16, a value Q(r_(i, j)) of a quantized residualsample of coordinates (i, j) may be calculated by adding up values ofmodified quantized residual samples starting from coordinates (0, j) tocoordinates (i, j).

Alternatively, using Equation 17 instead of Equation 16, a valueQ(r_(i, j)) of a quantized residual sample of coordinates (i, j) may becalculated.

$\begin{matrix}{{Q( r_{i,j} )} = \{ \begin{matrix}{r_{i,j}^{\prime},} & {{i = 0},} & {0 \leq j \leq ( {N - 1} )} \\{{r_{i,j}^{\prime} + {Q( r_{{({i - 1})},j} )}},} & {{1 \leq i \leq ( {M - 1} )},} & {0 \leq j \leq ( {N - 1} )}\end{matrix} } & \lbrack {{Equation}17} \rbrack\end{matrix}$

Equation 17 is the reverse process corresponding to Equation 4.According to Equation 17, a value Q(r_(0, j)) of a quantized residualsample of coordinates (0, j) is derived as a value r′_(0, j) of amodified quantized residual sample of coordinates (0, j). Q(r_(i, j)) ofother coordinates (i, j) is derived as the sum of a value r′_(i, j) of amodified quantized residual sample of coordinates (i, j) and a valueQ(r_(i−1, j)) of a quantized residual sample of coordinates (i−1, j).That is, a difference value r′_(i, j) is added up by using a valueQ(r_(i−1, j)) of a quantized residual sample of coordinates (i−1, j) asa prediction value, thereby deriving a quantized residual sample valueQ(r_(i, j)).

When the prediction direction of BDPCM is a vertical direction, theimage decoding apparatus may generate a quantized residual block from amodified quantized residual block by using Equation 18.

Q(r _(i,j))=Σ_(k=0) ^(j) r′ _(i,k), 0≤i≤(M−1), 0≤j≤(N−1)   [Equation 18]

As defined in Equation 18, a value Q(r_(i, j)) of a quantized residualsample of coordinates (i, j) may be calculated by adding up values ofmodified quantized residual samples starting from coordinates (i, 0) tocoordinates (i, j).

Alternatively, using Equation 19 instead of Equation 18, a valueQ(r_(i, j)) of a quantized residual sample of coordinates (i, j) may becalculated.

$\begin{matrix}{{Q( r_{i,j} )} = \{ \begin{matrix}{r_{i,j}^{\prime},} & {{0 \leq i \leq ( {M - 1} )},} & {j = 0} \\{{r_{i,j}^{\prime} + {Q( r_{i,{({j - 1})}} )}},} & {{0 \leq i \leq ( {M - 1} )},} & {1 \leq j \leq ( {N - 1} )}\end{matrix} } & \lbrack {{Equation}19} \rbrack\end{matrix}$

Equation 19 is the reverse process corresponding to Equation 15.According to Equation 19, a value Q(r_(i, 0)) of a quantized residualsample of coordinates (i, 0) is derived as a value r′_(i, 0) of amodified quantized residual sample of coordinates (i, 0). Q(r_(i, j)) ofother coordinates (i, j) is derived as the sum of a value r′_(i, j) of amodified quantized residual sample of coordinates (i, j) and a valueQ(r_(i, j−1)) of a quantized residual sample of coordinates (i, j−1).That is, a difference value r′_(i, j) is added up by using a valueQ(r_(i, j−1)) of a quantized residual sample of coordinates (i, j−1) asa prediction value, thereby deriving a quantized residual sample valueQ(r_(i, j)).

When a quantized residual block composed of quantized residual samplesis generated by performing step S3240 according to the above-describedmethod, the image decoding apparatus performs dequantization on thequantized residual block in step S3250 to generate a residual block ofthe current block. When BDPCM is applied, transformation of the currentblock is skipped as described above. Therefore, inverse transformationof a dequantized residual block may be skipped.

Afterward, the image decoding apparatus may reconstruct the currentblock in step S3260 on the basis of the prediction block generated instep S3220 and the residual block generated in step S3250. For example,the image decoding apparatus may reconstruct the current block (valuesof reconstructed samples) by adding the prediction block (values ofpredicted samples) and the residual block (values of residual samples).For example, a reconstructed sample value may be generated by adding adequantized quantized sample Q⁻¹(Q(r_(i,j))) to an intra blockprediction value. Differential encoding mode information indicatingwhether BDPCM is applied to the current block may be signaled through abitstream. In addition, when BDPCM is applied to the current block,differential encoding direction information indicating the predictiondirection of BDPCM may be signaled through a bitstream. When BDPCM isnot applied to the current block, the differential encoding directioninformation may not be signaled.

FIGS. 33 to 35 are views schematically showing syntax for signalinginformation on BDPCM.

FIG. 33 is a view showing syntax of a sequence parameter set accordingto an embodiment for signaling BDPCM information. In an embodiment, allSPS RBSPs included in at least one access unit (AU) having a value of 0as a temporal ID (TemporalId) or provided through external means may beset to be used before being referenced in a decoding process. Inaddition, an SPS NAL unit including an SPS RBSP may be set to havenuh_layer_id that is the same as nuh_layer_id of a PPS NAL unitreferring to the SPS NAL unit. In CVS, all SPS NAL units having aparticular sps_seq_parameter_set_id value may be set to have the samecontent. In the seq_parameter_set_rbsp( ) syntax of FIG. 33,sps_transform_skip_enable_flag, described above, andsps_bdpcm_enabled_flag, described later, are disclosed.

The syntax element sps_bdpcm_enabled_flag may indicate whether for anintra coding unit, intra_bdpcm_flag is provided in CU syntax. Forexample, a first value (e.g., 0) of the sps_bdpcm_enabled_flag mayindicate that for an intra coding unit, the intra_bdpcm_flag is notprovided in CU syntax. A second value (e.g., 1) of thesps_bdpcm_enabled_flag may indicate that for an intra coding unit, theintra_bdpcm_flag is provided in CU syntax. In the meantime, when thesps_bdpcm_enabled_flag is not provided, the value of thesps_bdpcm_enabled_flag may be set to the first value (e.g., 0).

FIG. 34 is a view showing an embodiment of syntax for signaling whetherlimitation on BDPCM is applied. In an embodiment, a predeterminedlimitation condition in the encoding/decoding process may be signaledusing general_constraint_info( ) syntax. Using the syntax of FIG. 34,the syntax element no_bdpcm_constraint_flag indicating whether the valueof the sps_bdpcm_enabled_flag described above needs to be set to 0 maybe signaled. For example, a first value (e.g., 0) of theno_bdpcm_constraint_flag may indicate that such a limitation is notapplied. When the value of the no_bdpcm_constraint_flag is a secondvalue (e.g., 1), the value of the sps_bdpcm_enabled_flag may be forcedto be a first value (e.g., 0).

FIG. 35 is a view showing an embodiment of coding unit( ) syntax forsignaling information on BDPCM for an encoding unit. As shown in FIG.35, the syntax elements intra_bdpcm_flag and intra_bdpcm_dir_flag may besignaled using coding_unit( ) syntax. The syntax elementintra_bdpcm_flag may indicate whether BDPCM is applied to a current lumaencoding block located at (x0, y0).

For example, a first value (e.g., 0) of the intra_bdpcm_flag mayindicate that BDPCM is not applied to the current luma encoding block. Asecond value (e.g., 1) of the intra_bdpcm_flag may indicate that BDPCMis applied to the current luma encoding block. By indicating that BDPCMis applied, the intra_bdpcm_flag may indicate whether transformation isskipped and also whether an intra luma prediction mode is performed bythe intra_bdpcm_dir_flag, which will be described later.

In the meantime, the value of the above-described variableBdpcmFlag[x][y] may be set to the value of the intra_bdpcm_flag, withrespect to x=x0..x0+cbWidth−1 and y=y0..y0+cbHeight−1.

The syntax element intra_bdpcm_dir_flag may indicate a predictiondirection of BDPCM. For example, a first value (e.g., 0) of theintra_bdpcm_dir_flag may indicate that the BDPCM prediction direction isa horizontal direction. A second value (e.g., 1) of theintra_bdpcm_dir_flag may indicate that the BDPCM prediction direction isa vertical direction.

In the meantime, the value of the variable BdpcmDir[x][y] may be set tothe value of the intra_bdpcm_dir_flag, with respect tox=x0..x0+cbWidth−1 and y=y0..y0+cbHeight−1.

Intra Prediction on Chroma Block

When intra prediction is performed on a current block, prediction on aluma component block (luma block) of the current block and prediction ona chroma component block (chroma block) may be performed. In this case,an intra prediction mode for the chroma block may be set separately froman intra prediction mode for the luma block.

For example, the intra prediction mode for the chroma block may beindicated on the basis of intra chroma prediction mode information. Theintra chroma prediction mode information may be signaled in the form ofa syntax element intra_chroma_pred_mode. For example, the intra chromaprediction mode information may indicate one of the following: a planarmode, a DC mode, a vertical mode, a horizontal mode, a derived mode(DM), and a cross-component linear model (CCLM) mode. Herein, the planarmode may refer to intra prediction mode 0, the DC mode may refer tointra prediction mode 1, the vertical mode may refer to intra predictionmode 26, and the horizontal mode may refer to intra prediction mode 10.The DM may also be referred to as a direct mode. The CCLM may also bereferred to as a linear model (LM). The CCLM mode may include any one ofthe following: L_CCLM, T_CCLM, and LT_CCLM.

In the meantime, the DM and the CCLM are dependent intra predictionmodes for predicting a chroma block by using information of a lumablock. The DM may refer to a mode in which the same intra predictionmode as the intra prediction mode for the luma component is applied asthe intra prediction mode for the chroma component. In addition, theCCLM may refer to an intra prediction mode in which in a process ofgenerating a prediction block for a chroma block, reconstructed samplesof a luma block are sub-sampled, and then samples derived by applyingCCLM parameters α and β to the sub-sampled samples are used asprediction samples of the chroma block.

Overview of Cross-Component Linear Model (CCLM) Mode

As described above, the CCLM mode may be applied to a chroma block. TheCCLM mode is the intra prediction mode in which the correlation betweena luma block and a chroma block corresponding to the luma block is used,and is performed by deriving a linear model on the basis of neighboringsamples of the luma block and neighboring samples of the chroma block.In addition, a prediction sample of the chroma block may be derived onthe basis of the derived linear model and the reconstructed samples ofthe luma block.

Specifically, when the CCLM mode is applied to a current chroma block,parameters for a linear model may be derived on the basis of neighboringsamples used for intra prediction of the current chroma block andneighboring samples used for intra prediction of a current luma block.For example, a linear model for a CCLM may be represented on the basisof the equation as below.

pred_(c)(i,j)=α·rec_(L)′(i,j)+β  [Equation 20]

Herein, pred_(c)(i,j) may refer to a prediction sample of coordinates(i,j) of the current chroma block in the current CU. rec_(L)′(i,j) mayrefer to a reconstructed sample of coordinates (i,j) of the current lumablock in the CU. For example, the rec_(L)′(i,j) may refer to adown-sampled reconstructed sample of the current luma block. Linearmodel coefficients α and β may be signaled, or may be derived from aneighboring sample.

Joint Coding of Residuals (Joint CbCr)

In an encoding/decoding process according to an embodiment, chromaresiduals may be encoded/decoded together. This may be referred to asjoint coding of residuals or as joint CbCr. Whether to apply (activate)a joint coding mode of CbCr may be signaled by a joint coding modesignaling flag tu_joint_cbcr_residual_flag that is signaled at the levelof the transformation basis. In addition, a selected encoding mode maybe derived by chroma CBFs. The flag tu_joint_cbcr_residual_flag may bepresent when a value of at least one chroma CBF for the transformationbasis is 1. A chroma QP offset value indicates a difference between ageneral chroma QP offset value signaled for a regular chroma residualencoding mode and a chroma QP offset value for a CbCr joint coding mode.The chroma QP offset value may be signaled through a PPS or a sliceheader. This QP offset value may be used to drive a chroma QP value forblocks using a joint chroma residual encoding mode.

In the case in which Mode 2, which is a corresponding joint chromaencoding mode, in the table below is activated for the transformationbasis, while quantization and decoding of the transformation basis areperformed, a chroma QP offset thereof may be added to a targetluma-derived chroma QP (applied luma-derived chroma QP).

Regarding other modes like Modes 1 and 3 in the table below, a chroma QPmay be derived in such a manner that it is obtained for a general Cb orCr block. Such a process of reconstructing chroma residuals (resCb andresCr) from a transformation block may be selected according to thetable below. When the present mode is activated, one single joint chromaresidual block (resJointC[x][y] in the table below) is signaled, and aresidual block resCb for a Cb and a residual block resCr for a Cr may bederived considering information such as tu_cbf_cb, tu_cbf_cr, and CSignthat is a sign value disclosed in a slice header.

In the encoding apparatus, a joint chroma component may be derived asfollows. According to a joint coding mode, resJointC{1, 2} may begenerated according to the following order. In the case of Mode 2(single residual with reconstruction Cb=C, Cr=CSign*C), a joint residualmay be determined according to the equation below.

resJointC[x][y]=(resCb[x][y]+CSign*resCr[x][y])/2.   [Equation 21]

Alternatively, in the case of Mode 1 (single residual withreconstruction Cb=C, Cr=(CSign*C)/2), a joint residual may be determinedaccording to the equation below.

resJointC[x][y]=(4*resCb[x][y]+2*CSign*resCr[x][y])/5.   [Equation 22]

Alternatively, in the case of Mode 3 (single residual withreconstruction Cr=C, Cb=(CSign*C)/2), a joint residual may be determinedaccording to the equation below.

resJointC[x][y]=(4*resCr[x][y]+2*CSign*resCb[x][y])/5.   [Equation 23]

TABLE 2 tu_cbf_cb tu_cbf_cr reconstruction of Cb and Cr residuals mode 10 resCb[ x ][ y ] = resJointC[ x ][ y ] 1 resCr[ x ][ y ] = ( CSign *resJointC[ x ][ y ] ) >> 1 1 1 resCb[ x ][ y ] = resJointC[ x ][ y ] 2resCr[ x ][ y ] = CSign * resJointC[ x ][ y ] 0 1 resCb[ x ][ y ] = (CSign * resJointC[ x ][ y ] ) >> 1 3 resCr[ x ][ y ] = resJointC[ x ][ y]

The above table shows reconstruction of chroma residuals. CSign refersto a sign value +1 or −1 specified in a slice header. resJointC[ ][ ]refers to a transmitted residual. In the table, the modes refer toTuCResMode, which will be described later. The three joint chromaencoding modes in the table may be supported only for an I slice. For Pand B slices, only mode 2 may be supported. Therefore, for P and Bslices, the syntax element tu_joint_cbcr_residual_flag may be providedonly when both chroma cbf (e.g., tu_cbf_cb and tu_cbf_cr) values are 1.In the meantime, in context modeling of tu_cbf_luma and tu_cbf_cb, atransformation depth may be removed.

Embodiment 1: OP Update Method Using ACT Qp Offset

As described above, the update of the QP to apply ACT may be performed.The above-described update of the QP has several problems. For example,when the above-described method is used, it is impossible to setdifferent ACT Qp offsets for individual color components. Further, thederived qP value may have a negative value. Accordingly, in thefollowing embodiment, described is a method of applying clipping to a Qpvalue derived on the basis of an ACT QP offset value of a colorcomponent value.

In an embodiment, a quantization parameter qP may be derived as follows.

First, when the cIdx has a value of 0, the qP and the ACT Qp offset maybe derived as shown in the equation below.

qP=Qp′_(Y)

ActQpOffset=5   [Equation 24]

Alternatively, when the TuCResMode[xTbY][yTbY] has a value of 2, the qPand the ACT Qp offset may be derived as shown in the equation below.

qP=Qp′_(CbCr)

ActQpOffset=5   [Equation 25]

Alternatively, when the cIdx has a value of 1, the qP and the ACT Qpoffset may be derived as shown in the equation below.

qP=Qp′_(Cb)

ActQpOffset=3   [Equation 26]

The quantization parameter qP may be updated as follows.

When the transform_skip_flag[xTbY][yTbY][cIdx] has a value of 0, the qPmay be derived as shown in the equation below.

qP=Max(0, qP−(cu_act_enabled_flag[xTbY][yTbY]?ActQpOffset:0))  [Equation 27]

Alternatively, when the transform_skip_flag[xTbY][yTbY][cIdx] has avalue of 1, the qP may be derived as shown in the equation below.

qP=Max(0, Max(QpPrimeTsMin,qP)−(cu_act_enabled_flag[xTbY][yTbY]?ActQpOffset:0))   [Equation 28]

In another embodiment, when the transform_skip_flag[xTbY][yTbY][cIdx]has a value of 1, the qP may be clipped using a value of theQpPrimeTsMin instead of 0 as shown in the equation below.

qP=Max(QpPrimeTsMin, qP−(cu_act_enabled_flag[xTbY][yTbY]?ActQpOffset:0)  [Equation 29]

In the meantime, in another embodiment, a quantization parameter qP maybe derived as follows.

First, when the cIdx has a value of 0, the qP and the ACT Qp offset maybe derived as shown in the equation below.

qP=Qp′_(Y)

ActQpOffset=5   [Equation 30]

Alternatively, when the TuCResMode[xTbY][yTbY] has a value of 2, the qPand the ACT Qp offset may be derived as shown in the equation below.

qP=Qp′_(CbCr)

ActQpOffset=5   [Equation 31]

Alternatively, when the cIdx has a value of 1, the qP and the ACT Qpoffset may be derived as shown in the equation below.

qP=Qp′_(Cb)

ActQpOffset=5   [Equation 32]

Alternatively, when the cIdx has a value of 2, the qP and the ACT Qpoffset may be derived as shown in the equation below.

qP=Qp′_(Cr)

ActQpOffset=3   [Equation 33]

The quantization parameter qP may be updated as follows.

When the transform_skip_flag[xTbY][yTbY][cIdx] has a value of 0, the qPmay be derived as shown in the equation below.

qP=Max(0, qP−(cu_act_enabled_flag[xTbY][yTbY]?ActQpOffset:0))  [Equation 34]

Alternatively, when the transform_skip_flag[xTbY][yTbY][cIdx] has avalue of 1, the qP may be derived as shown in the equation below.

qP=Max(0, Max(QpPrimeTsMin,qP)−(cu_act_enabled_flag[xTbY][yTbY]?ActQpOffset:0))   [Equation 35]

In another embodiment, when the transform_skip_flag[xTbY][yTbY][cIdx]has a value of 1, the qP may be clipped using a value of theQpPrimeTsMin instead of 0 as shown in the equation below.

qP=Max(QpPrimeTsMin, qP−(cu_act_enabled_flag[xTbY][yTbY]?ActQpOffset:0))  [Equation 36]

In the meantime, in still another embodiment, a quantization parameterqP may be derived as follows.

First, when the cIdx has a value of 0, the qP and the ACT Qp offset maybe derived as shown in the equation below.

qP=Qp′_(Y)

ActQpOffset=−5   [Equation 37]

Alternatively, when the TuCResMode[xTbY][yTbY] has a value of 2, the qPand the ACT Qp offset may be derived as shown in the equation below.

qP=Qp′_(CbCr)

ActQpOffset=−5   [Equation 38]

Alternatively, when the cIdx has a value of 1, the qP and the ACT Qpoffset may be derived as shown in the equation below.

qP=Qp′_(Cb)

ActQpOffset=−5   [Equation 39]

Alternatively, when the cIdx has a value of 2, the qP and the ACT Qpoffset may be derived as shown in the equation below.

qP=Qp′_(Cr)

ActQpOffset=−3   [Equation 40]

The quantization parameter qP may be updated as follows.

When the transform_skip_flag[xTbY][yTbY][cIdx] has a value of 0, the qPmay be derived as shown in the equation below.

qP=Max(0, qP+(cu_act_enabled_flag[xTbY][yTbY]?ActQpOffset:0))  [Equation 41]

Alternatively, when the transform_skip_flag[xTbY][yTbY][cIdx] has avalue of 1, the qP may be derived as shown in the equation below.

qP=Max(0, Max(QpPrimeTsMin,qP)+(cu_act_enabled_flag[xTbY][yTbY]?ActQpOffset:0))   [Equation 42]

In the meantime, in another embodiment, when thetransform_skip_flag[xTbY][yTbY][cIdx] has a value of 1, the qP may beclipped using a value of the QpPrimeTsMin instead of 0 as shown in theequation below.

qP=Max(QpPrimeTsMin, qP+(cu_act_enabled_flag[xTbY][yTbY]?ActQpOffset:0))  [Equation 43]

In the above description, Y, Cb, and Cr may denote three colorcomponents. For example, in ACT transformation, Y may correspond to C0.Cb may correspond to C1 or Cg. In addition, Cr may correspond to C2 orCo.

In addition, the ACTQpOffset values of −5, −5, and −3 for the threecolor components may be replaced with other values or other variables.

Embodiment 2: Signaling of OP Offset Adjustment for ACT

In the above-described embodiment, ACT QP offset adjustments are fixedto −5, −5, and −3 for Y, Cg, and Co components. In the presentembodiment, in order to provide more flexibility to an ACT QP adjustmentoffset, a method of signaling an ACT QP offset will be described. TheACT QP offset may be signaled as a parameter in a PPS.

In an embodiment, qp_offset may be signaled according to a syntax tableof FIG. 36. The syntax elements therefor are as follows.

A syntax element pps_act_qp_offsets_present_flag may indicate whether asyntax element related to an ACT QP offset is present in a PPS. Forexample, the pps_act_qp_offsets_present_flag may indicate whether syntaxelements pps_act_y_qp_offset, pps_act_cb_qp_offset, andpps_act_cr_qp_offset, which will be described later, are signaled as aPPS.

For example, a first value (e.g., 0) of thepps_act_qp_offsets_present_flag may indicate that thepps_act_y_qp_offset, the pps_act_cb_qp_offset, and thepps_act_cr_qp_offset are not signaled through a PPS syntax table.

A second value (e.g., 1) of the pps_act_qp_offsets_present_flag mayindicate that the pps_act_y_qp_offset, the pps_act_cb_qp_offset, and thepps_act_cr_qp_offset are signaled through a PPS syntax table.

When the pps_act_qp_offsets_present_flag is not provided from abitstream, the pps_act_qp_offsets_present_flag may be derived as thefirst value (e.g., 0). For example, when a flag (e.g.,sps_act_enabled_flag signaled in an SPS) indicating whether ACT isapplied has a first value (e.g., 0) indicating that ACT is not applied,the pps_act_qp_offsets_present_flag may be forced to have a first value(e.g., 0).

When a value of a syntax element cu_act_enabled_flag is a second value(e.g., 1) indicating that ACT is applied for the current encoding basis,syntax elements pps_act_y_qp_offset_plus5, pps_act_cb_qp_offset_plus5s,and pps_act_cr_qp_offset_plus3 may be used to determine offsets appliedto quantization parameter values qP for luma, Cb, and Cr components,respectively. When the values of the pps_act_y_qp_offset_plus5, thepps_act_cb_qp_offset_plus5, and the pps_act_cr_qp_offset_plus3 are notpresent in a bitstream, each value may be set to 0.

According to the syntax elements, a value of a variable PpsActQpOffsetYmay be determined to be pps_act_y_qp_offset_plus5-5. A value of avariable PpsActQpOffsetCb may be determined to bepps_act_cb_qp_offset_plus5-5. In addition, a value of a variablePpsActQpOffsetCr may be determined to be pps_act_cb_qp_offset_plus3-3.

Herein, ACT is not orthonormal transformation, so 5, 5, and 3 may beapplied as the constant offset values to be subtracted. In anembodiment, for bitstream matching, the values of the PpsActQpOffsetY,the PpsActQpOffsetCb, and the PpsActQpOffsetCr may have values rangingfrom −12 to 12. In addition, according to an embodiment, in addition to5, 5, and 3, the Qp offset values may be replaced with other constantvalues and used.

In another embodiment, a QP may be adjusted using a more flexible ACT_QPoffset. In the following embodiment, described is an example in which anACT QP offset is signaled in a bitstream. Accordingly, the ACT QP offsetmay have a wider offset range. Therefore, the QP updated using the ACTQP offset is more likely to be out of an available range, so it isnecessary to perform clipping on the upper and lower limits for theupdated QP (more detailed embodiments will be described later inEmbodiments 6 and 7)

Variables PpsActQpOffsetY, PpsActQpOffsetCb, PpsActQpOffsetCr, andPpsActQpOffsetCbCr indicating ACT QP offsets may be values derived usingACT QP offsets signaled through a bitstream, or preset constants. Forbitstream conformance, the PpsActQpOffsetY, the PpsActQpOffsetCb, thePpsActQpOffsetCr, and the PpsActQpOffsetCbCr may have values rangingfrom −12 to +12.

When without using a fixed value, a value of a QP offset is signaled andits value has a value ranging from −12 to 12, it is necessary to clipthe upper limit value of the derived QP value, in addition to clippingthe lower limit value of the QP value derived to avoid an QP having anegative value.

To prevent the value of the qP from having a negative value, the lowestvalue of the qP may be forced to 0. Alternatively, the lowest value ofthe qP may be set to the value determined by the signaled syntaxelement. For example, to signal the lowest value of the qP when atransformation skip mode is applied, a syntax element QpPrimeTsMinindicating a value of the qP applied when the transformation skip modeis applied may be used. The maximum value of the qP may be limited tothe available maximum value (e.g., 63) of the maximum available qP valuedetermined according to a signaled syntax element.

In an embodiment according to the above, a quantization parameter qP maybe derived as follows. First, when the cIdx has a value of 0, the qP andthe ACT Qp offset may be derived as shown in the equation below.

qP=Qp′_(Y)

ActQpOffset=PpsActQpOffsetY   [Equation 44]

Alternatively, when the TuCResMode[xTbY][yTbY] has a value of 2, the qPand the ACT Qp offset may be derived as shown in the equation below.

qP=Qp′_(CbCr)

ActQpOffset=PpsActQpOffsetCbCr   [Equation 45]

Alternatively, when the cIdx has a value of 1, the qP and the ACT Qpoffset may be derived as shown in the equation below.

qP=Qp′_(Cb)

ActQpOffset=PpsActQpOffsetCb   [Equation 46]

Alternatively, when the cIdx has a value of 2, the qP and the ACT Qpoffset may be derived as shown in the equation below.

qP=Qp′_(Cr)

ActQpOffset=PpsActQpOffsetCr   [Equation 47]

In an embodiment, the quantization parameter qP may be updated asfollows. When the transform_skip_flag[xTbY][yTbY][cIdx] has a value of0, the qP may be derived as shown in the equation below.

qP=Clip3(0, 63, qP−(cu_act_enabled_flag[xTbY][yTbY]?ActQpOffset:0))  [Equation 48]

Alternatively, when the transform_skip_flag[xTbY][yTbY][cIdx] has avalue of 1, the qP may be derived as shown in the equation below.

qP=Clip3(0, 63, Max(QpPrimeTsMin,qP)−(cu_act_enabled_flag[xTbY][yTbY]?ActQpOffset:0)   [Equation 49]

In another embodiment, when the transform_skip_flag[xTbY][yTbY][cIdx]has a value of 1, the lowest value of the qP may be clipped using avalue of the QpPrimeTsMin instead of 0 as shown in the equation below.

[Equation 50]

The quantization parameter qP may be updated as follows.

When the transform_skip_flag[xTbY][yTbY][cIdx] has a value of 0, the qPmay be derived as shown in the equation below.

qP=Clip3(0, 63, qP−(cu_act_enabled_flag[xTbY][yTbY]?ActQpOffset:0))

Alternatively, when the transform_skip_flag[xTbY][yTbY][cIdx] has avalue of 1, the qP may be derived as shown in the equation below.

qP=Clip3(QpPrimeTsMin, 63,qP−cu_act_enabled_flag[xTbY][yTbY]?ActQpOffset:0)

In another embodiment, the quantization parameter qP may be updated asfollows.

When the transform_skip_flag[xTbY][yTbY][cIdx] has a value of 0, the qPmay be derived as shown in the equation below.

qP=Clip3(0, 63+QpBdOffset,qP+(cu_act_enabled_flag[xTbY][yTbY]?ActQpOffset:0))   [Equation 51]

Alternatively, when the transform_skip_flag[xTbY][yTbY][cIdx] has avalue of 1, the qP may be derived as shown in the equation below.

qP=Clip3(0, 63+QpBdOffset, Max(QpPrimeTsMin,qP)+(cu_act_enabled_flag[xTbY][yTbY]?ActQpOffset:0)

In another embodiment, when the transform_skip_flag[xTbY][yTbY][cIdx]has a value of 1, the lowest value of the qP may be clipped using avalue of the QpPrimeTsMin instead of 0 as shown in the equation below.

[Equation 53]

The quantization parameter qP may be updated as follows.

When the transform_skip_flag[xTbY][yTbY][cIdx] has a value of 0, the qPmay be derived as shown in the equation below.

qP=Clip3(0, 63+QpBdOffset,qP+(cu_act_enabled_flag[xTbY][yTbY]?ActQpOffset:0))

Alternatively, when the transform_skip_flag[xTbY][yTbY][cIdx] has avalue of 1, the qP may be derived as shown in the equation below.

qP=Clip3(QpPrimeTsMin, 63+QpBdOffset,qP+cu_act_enabled_flag[xTbY][yTbY]?ActQpOffset:0)

Embodiment 3: Method to Allow ACT When Chroma BDPCM is Performed

In an embodiment, when BDPCM is applied to a luma component block, ACTmay be applied to encode/decode the block. However, when BDPCM isapplied to a chroma component block, ACT is limited not to be applied toencode/decode the block.

In the meantime, even when BDPCM is applied to a chroma component block,ACT is applied to the block, thereby improving encoding efficiency. FIG.37 shows an embodiment of a syntax configuration for applying ACT evenwhen BDPCM is applied to a chroma component block. As shown in FIG. 37,by removing the condition for obtaining a BDPCM syntax element for achroma component according to the value of the cu_act_enabled_flagindicating whether ACT is applied to the current encoding basis, theBDPCM syntax element therefor may be obtained regardless of whether ACTis applied to a chroma component block, and BDCPM encoding may beperformed accordingly.

Embodiment 4: Method of Applying ACT Even When Encoding/Decoding isPerformed with CCLM

Both CCLM and ACT are intended to remove unnecessary overlap betweencomponents. There are some overlapping parts between CCLM and ACT, buteven after applying all of these, it is impossible to completely removeoverlap between components. Therefore, overlap between components may bemore removed by applying CCLM and ACT together.

The following embodiment describes an embodiment in which CCLM and ACTare applied together. In performing decoding, the decoding apparatus mayapply CCLM first, and apply ACT. When ACT is applied to both BDPCM andCCLM for a chroma component, a syntax table for signaling this may bemodified as shown in FIG. 38. Accordingly, as shown in the syntax tableof FIG. 38, among the limitations for signaling a syntax element relatedto intra_bdpcm_chroma and cclm, if(!cu_act_enabled_flag) for signaling asyntax element depending on whether ACT is not applied may be removedfrom the syntax table.

Embodiment 5: Method of Applying Flexible ACT Qp Including Joint CbCr

When an ACT mode is applied, a prediction residual may be transformedfrom one color space (e.g., GBR or YCbCr) into a YCgCo color space. Inaddition, residuals of the transformation basis may be encoded in theYCgCo color space. As an embodiment of ACT core transformation(transformation kernel) used for color space transformation, thefollowing transformation kernel as described above may be used.

$\begin{matrix}{\begin{bmatrix}C_{0}^{\prime} \\C_{1}^{\prime} \\C_{2}^{\prime}\end{bmatrix} = {{\begin{bmatrix}2 & 1 & 1 \\2 & {- 1} & {- 1} \\0 & {- 2} & 2\end{bmatrix}\begin{bmatrix}C_{0} \\C_{1} \\C_{2}\end{bmatrix}}/4}} & \lbrack {{Equation}54} \rbrack\end{matrix}$ $\begin{matrix}{\begin{bmatrix}C_{0} \\C_{1} \\C_{2}\end{bmatrix} = {\begin{bmatrix}1 & 1 & 0 \\1 & {- 1} & {- 1} \\1 & {- 1} & 1\end{bmatrix}\begin{bmatrix}C_{0}^{\prime} \\C_{1}^{\prime} \\C_{2}^{\prime}\end{bmatrix}}} & \lbrack {{Equation}55} \rbrack\end{matrix}$

As described in the equations above, C0′, C1′, and C2′ (herein, C0′=Y,C1′=Cg, C2′=Co) transformations are not normalized. For example, the L2norm does not have a value of 1. For example, the L2 norm oftransformation for an individual component may have a value of about 0.6for C0′ and C1′, and may have a value of about 0.7 for C2′. Herein, theL2 norm is a value obtained as the square root of the sum of the squaresof respective coefficients. For example, C0′=2/4*C0+1/4*C1+1/4*C2 may becalculated. Therefore, the norm of C0′ may be calculated as the squareroot of (2/4*2/4+1/4*1/4+1/4*1/4). Therefore, this may be calculated asthe square root of 6/16, and may be calculated as having a value ofabout 0.6.

When normalized transformation is not applied, the dynamic range of theindividual component is irregular. In addition, this causes a decreasein encoding performance of a general video compression system.

In order to compensate for the dynamic range of a residual signal, a QPoffset value for compensating a dynamic range change for an individualtransformation component is transmitted, so that QP adjustment may beperformed. For example, this embodiment may be applied to a general QPadjustment control method for ACT transformation as well as joint CbCr.

Individual color components are not encoded independently, but together,so that the method described above in Embodiment 3 for joint CbCr maycause a dynamic range change between the individual color components.

In encoding and decoding methods according to an embodiment, ACT QPoffset adjustment may be fixed to −5, which may be equally applied to Y,Cg, and Co.

In an embodiment, in order to provide flexible Qp control for individualcomponents and jointCbCr, it may be allowed to use different ACT Qpoffsets for Y, Cb, Cr, and/or joint CbCr. The ACT Qp offset values maybe determined on the basis of a component index, and/or joint CbCr,and/or a joint CbCr mode.

In order to indicate the ACT Qp offsets, ppsActQpOffsetY,ppsActQpOffsetCb, and ppsActQpOffsetCr may be used. In addition,ppsActQpOffsetCbCr may be used for the ACT QP offset of joint CbCr mode2 having CBF in which all Cb and Cr components have non-zero values.These values (e.g., ppsActQpOffsetY, ppsActQpOffsetCb, ppsActQpOffsetCr,and ppsActQpOffsetCbCr) may be pre-determined to be predeterminedvalues, or signaled through a bitstream. The ACT QP offset of the jointCbCr mode may be set in another method or to another value.

In an embodiment, the ACT Qp offsets of −5, −5, and −3 may be used forY, Cb, and Cr, and the ACT Qp offset of −4 may be used for joint CbCr.

In another embodiment, the ACT Qp offsets of −5, −4, and −3 may be usedfor Y, Cb, and Cr, and the ACT Qp offset of −3 may be used for the jointCbCr mode in which the value of tu_cbf_cb is not 0.

In still another embodiment, the ACT QP offset of joint CbCr mode 2 mayhave its own offset value. For another joint CbCr mode, the ACT QPoffset may use the offset of a corresponding component. For example, aquantization parameter qP may be determined as follows. First, when thecIdx has a value of 0, the qP and the ACT Qp offset may be derived asshown in the equation below.

qP=Qp′_(Y)

ActQpOffset=ppsActQpOffsetY   [Equation 56]

Alternatively, when the TuCResMode[xTbY][yTbY] has a value of 2, the qPand the ACT Qp offset may be derived as shown in the equation below.

qP=Qp′_(CbCr)

ActQpOffset=ppsActQpOffsetCbCr   [Equation 57]

Alternatively, when the cIdx has a value of 1, the qP and the ACT Qpoffset may be derived as shown in the equation below.

qP=Qp^(Cb)

ActQpOffset=ppsActQpOffsetCb   [Equation 58]

Alternatively, when the cIdx has a value of 2, the qP and the ACT Qpoffset may be derived as shown in the equation below.

qP=Qp′_(Cr)

ActQpOffset=ppsActQpOffsetCr   [Equation 59]

In an embodiment, the quantization parameter qP may be updated asfollows.

When the transform_skip_flag[xTbY][yTbY][cIdx] has a value of 0, the qPmay be derived as shown in the equation below.

qP=Clip3(0, 63+QpBdOffset,qP+(cu_act_enabled_flag[xTbY][yTbY]?ActQpOffset:0))   [Equation 60]

Alternatively, when the transform_skip_flag[xTbY][yTbY][cIdx] has avalue of 1, the qP may be derived as shown in the equation below.

qP=Clip3(QpPrimeTsMin, 63+QpBdOffset,qP+cu_act_enabled_flag[xTbY][yTbY]?ActQpOffset:0)   [Equation 61]

In another embodiment, with respect to a joint CbCr mode (e.g., in thecase of Modes 1 and 2) with tu_cbf_cb!=0, an offset for joint CbCr maybe determined using the ppsActQpOffsetCb. Alternatively, with respect toa joint CbCr mode (e.g., in the case of Mode 3) with tu_cbf_cb==0, anoffset for joint CbCr may be determined using the ppsActQpOffsetCr. Forexample, the above-described embodiment may be modified and applied asfollows.

The quantization parameter qP may be updated as follows. First, when thecIdx has a value of 0, the qP and the ACT Qp offset may be derived asshown in the equation below.

qP=Qp′_(Y)

ActQpOffset=ppsActQpOffsetY   [Equation 62]

Alternatively, when the TuCResMode[xTbY][yTbY] has a value of 2, the qPmay be derived as shown in the equation below.

qP=Qp′_(CbCr)   [Equation 63]

Alternatively, when the cIdx has a value of 1, the qP and the ACT Qpoffset may be derived as shown in the equation below.

qP=Qp′_(Cb)

ActQpOffset=ppsActQpOffsetCb   [Equation 64]

Alternatively, when the cIdx has a value of 2, the qP and the ACT Qpoffset may be derived as shown in the equation below.

qP=Qp′_(Cr)

ActQpOffset=ppsActQpOffsetCr   [Equation 65]

When the cIdx does not have a value of 0 and the TuCResMode[xTbY][yTbY]does not have a value of 0, the ACT Qp offset for the joint CbCr modemay be determined according to the pseudocode below.

if (TuCResMode[xTbY][yTbY] is equal to 1 or 2)

ActQpOffset=ppsActQpOffsetCb;

else

ActQpOffset=ppsActQpOffsetCr;   [Equation 66]

In an embodiment, the quantization parameter qP may be updated asfollows. When the transform_skip_flag[xTbY][yTbY][cIdx] has a value of0, the qP may be derived as shown in the equation below.

qP=Clip3(0, 63+QpBdOffset,qP+(cu_act_enabled_flag[xTbY][yTbY]?ActQpOffset:0))   [Equation 67]

Alternatively, when the transform_skip_flag[xTbY][yTbY][cIdx] has avalue of 1, the qP may be derived as shown in the equation below.

qP=Clip3(QpPrimeTsMin, 63+QpBdOffset,qP+cu_act_enabled_flag[xTbY][yTbY]?ActQpOffset:0)   [Equation 68]

In another embodiment, regardless of the joint CbCr mode,ppsActQpOffsetY is used when the component index is Y, ppsActQpOffsetCbis used when the component index is Cb, and ppsActQpOffsetCr is usedwhen the component index is Cr, whereby the qP may be derived. Forexample, the quantization parameter qP may be derived as follows.

First, when the cIdx has a value of 0, the qP and the ACT Qp offset maybe derived as shown in the equation below.

qP=Qp′_(Y)

ActQpOffset=ppsActQpOffsetY   [Equation 69]

Alternatively, when the TuCResMode[xTbY][yTbY] has a value of 2, the qPand the ACT Qp offset may be derived as shown in the equation below.

qP=Qp′_(CbCr)

ActQpOffset=(cIdx==1)?ppsActQpOffsetCb:ppsActQpOffsetCr   [Equation 70]

Alternatively, when the cIdx has a value of 1, the qP and the ACT Qpoffset may be derived as shown in the equation below.

qP=Qp′_(Cb)

ActQpOffset=ppsActQpOffsetCb   [Equation 71]

Alternatively, when the cIdx has a value of 2, the qP and the ACT Qpoffset may be derived as shown in the equation below.

qP=Qp′_(Cr)

ActQpOffset=ppsActQpOffsetCr   [Equation 72]

The quantization parameter qP may be updated as follows.

When the transform_skip_flag[xTbY][yTbY][cIdx] has a value of 0, the qPmay be derived as shown in the equation below.

qP=Clip3(0, 63+QpBdOffset,qP+(cu_act_enabled_flag[xTbY][yTbY]?ActQpOffset:0))   [Equation 73]

Alternatively, when the transform_skip_flag[xTbY][yTbY][cIdx] has avalue of 1, the qP may be derived as shown in the equation below.

qP=Clip3(QpPrimeTsMin, 63+QpBdOffset,qP+cu_act_enabled_flag[xTbY][yTbY]?ActQpOffset:0)   [Equation 74]

Embodiment 6: Method of Signaling ACT Qp Offset Including Joint CbCr

Hereinafter, described is an example in which an ACT QP offset issignaled through a bitstream to provide more flexibility. ACT QP offsetsmay be signaled through a SPS, a PPS, a picture header, a slice header,or header sets of other types. An ACT Qp offset of joint CbCr may besignaled separately, or may be derived from ACT QP offsets for Y, Cb,and Cr.

Without loss of generality, FIG. 39 shows an example of a syntax tablein which an ACT Qp offset is signaled in a PPS. As in the embodiment ofFIG. 39, one ACT Qp offset may be signaled for joint CbCr. The syntaxelements indicated in the syntax table of FIG. 39 will be described.

A syntax element pps_act_qp_offsets_present_flag may indicate whether asyntax element related to an ACT QP offset is present in a PPS. Forexample, the pps_act_qp_offsets_present_flag may indicate whether syntaxelements pps_act_y_qp_offset_plusX1, pps_act_cb_qp_offset_plusX2,pps_act_cr_qp_offset_plusX3, and pps_act_cbcr_qp_offset_plusX4, whichwill be described later, are signaled as a PPS.

For example, a first value (e.g., 0) of thepps_act_qp_offsets_present_flag may indicate that thepps_act_y_qp_offset_plusX1, the pps_act_cb_qp_offset_plusX2, thepps_act_cr_qp_offset_plusX3, and the pps_act_cbcr_qp_offset_plusX4 arenot signaled through a PPS syntax table.

A second value (e.g., 1) of the pps_act_qp_offsets_present_flag mayindicate that the pps_act_y_qp_offset_plusX1, thepps_act_cb_qp_offset_plusX2, the pps_act_cr_qp_offset_plusX3, and thepps_act_cbcr_qp_offset_plusX4 are signaled through a PPS syntax table.

When the pps_act_qp_offsets_present_flag is not provided from abitstream, the pps_act_qp_offsets_present_flag may be derived as thefirst value (e.g., 0). For example, when a flag (e.g.,sps_act_enabled_flag signaled in an SPS) indicating whether ACT isapplied has a first value (e.g., 0) indicating that ACT is not applied,the pps_act_qp_offsets_present_flag may be forced to have a first value(e.g., 0).

When a value of a syntax element cu_act_enabled_flag is a second value(e.g., 1) indicating that ACT is applied for the current encoding basis,the syntax elements pps_act_y_qp_offset_plusX1,pps_act_cb_qp_offset_plusX2, pps_act_cr_qp_offset_plusX3, andpps_act_cbcr_qp_offset_plusX4 may be used to determine offsets appliedto quantization parameter values qP for luma, Cb, Cr components, and ajoint CbCr component, respectively. When the values of thepps_act_y_qp_offset_plusX1, the pps_act_cb_qp_offset_plusX2, thepps_act_cr_qp_offset_plusX3, and the pps_act_cbcr_qp_offset_plusX4 arenot present in a bitstream, each value may be set to 0.

According to the syntax elements, values of variables PpsActQpOffsetY,PpsActQpOffsetCb, PpsActQpOffsetCr, and PpsActQpOffsetCbCr may bedetermined as shown in the equation below.

PpsActQpOffsetY=pps_act_y_qp_offset_plusX1-X1

PpsActQpOffsetCb=pps_act_cb_qp_offset_plusX2-X2

PpsActQpOffsetCr=pps_act_cr_qp_offset_plusX3-X3

PpsActQpOffsetCbCr=pps_act_cbcr_qp_offset_plusX4-X4   [Equation 75]

Herein, X1, X2, X3, and X4 may indicate predetermined constant values.These may be the same values or different values, or only some may havethe same value.

In an embodiment, for bitstream matching, the values of thePpsActQpOffsetY, the PpsActQpOffsetCb, the PpsActQpOffsetCr, and thePpsActQpOffsetCbCr may be limited to have values ranging from −12 to 12.

According to determination of the variables, a quantization parameter qPmay be determined as follows. First, when the cIdx has a value of 0, theqP and the ACT Qp offset may be derived as shown in the equation below.

qP=Qp′_(Y)

ActQpOffset=PpsActQpOffsetY   [Equation 76]

Alternatively, when the TuCResMode[xTbY][yTbY] has a value of 2, the qPand the ACT Qp offset may be derived as shown in the equation below.

qP=Qp′_(CbCr)

ActQpOffset=PpsActQpOffsetCbCr   [Equation 77]

Alternatively, when the cIdx has a value of 1, the qP and the ACT Qpoffset may be derived as shown in the equation below.

qP=Qp′_(Cb)

ActQpOffset=PpsActQpOffsetCb   [Equation 78]

Alternatively, when the cIdx has a value of 2, the qP and the ACT Qpoffset may be derived as shown in the equation below.

qP=Qp′_(Cr)

ActQpOffset=PpsActQpOffsetCr   [Equation 79]

In another embodiment of signaling an ACT Qp offset, a plurality of ACTQP offsets may be signaled for different joint CbCr modes identified asmode A and mode B.

The joint CbCr mode A may refer to a jointCbCr mode having tu_cbf_cbhaving a non-zero value, such as Mode 1 and Mode 2 of Table 2 describedabove. The joint CbCr mode B may refer to a jointCbCrmode havingtu_cbf_cb having a value of 0, such as Mode 3 of Table 2 describedabove. The syntax table changed accordingly is shown in FIG. 40. Thesyntax elements indicated in the syntax table of FIG. 40 will bedescribed.

When a value of a syntax element cu_act_enabled_flag is a second value(e.g., 1) indicating that ACT is applied for the current encoding basis,syntax elements pps_act_y_qp_offset_plusX1, pps_act_cb_qp_offset_plusX2,pps_act_cr_qp_offset_plusX3, pps_act_cbcr_qp_offset_modeA_plusX4, andpps_act_cbcr_qp_offset_modeB_plusX5 may be used to determine offsetsapplied to quantization parameter values qP for luma, Cb, Cr components,and a joint CbCr component, respectively. When the values of thepps_act_y_qp_offset_plusX1, the pps_act_cb_qp_offset_plusX2, thepps_act_cr_qp_offset_plusX3, the pps_act_cbcr_qp_offset_modeA_plusX4,and the pps_act_cbcr_qp_offset_modeB_plusX5 are not present in abitstream, each value may be set to 0.

According to the syntax elements, values of variables PpsActQpOffsetY,PpsActQpOffsetCb, PpsActQpOffsetCr, PpsActQpOffsetCbCrModeA, andPpsActQpOffsetCbCrModeB may be determined as shown in the equationbelow.

PpsActQpOffsetY=pps_act_y_qp_offset_plusX1-X1

PpsActQpOffsetCb=pps_act_cb_qp_offset_plusX2-X2

PpsActQpOffsetCr=pps_act_cr_qp_offset_plusX3-X3

PpsActQpOffsetCbCrModeA=pps_act_cbcr_qp_offset_modeA_plusX4-X4

PpsActQpOffsetCbCrModeB=pps_act_cbcr_qp_offset_modeB_plusX5-X5  [Equation 80]

Herein, X1, X2, X3, X4, and X5 may indicate predetermined constantvalues. These may be the same values or different values, or only somemay have the same value. In an embodiment, for bitstream matching, thevalues of the PpsActQpOffsetY, the PpsActQpOffsetCb, thePpsActQpOffsetCr, the PpsActQpOffsetCbCrModeA, and thePpsActQpOffsetCbCrModeB may be limited to have values ranging from −12to 12.

According to determination of the variables, a quantization parameter qPmay be determined as follows. First, when the cIdx has a value of 0, theqP and the ACT Qp offset may be derived as shown in the equation below.

qP=Qp′_(Y)

ActQpOffset=PpsActQpOffsetY   [Equation 81]

Alternatively, when the TuCResMode[xTbY][yTbY] has a value of 2, the qPmay be derived as shown in the equation below.

qP=Qp′_(CbCr)   [Equation 82]

Alternatively, when the cIdx has a value of 1, the qP and the ACT Qpoffset may be derived as shown in the equation below.

qP=Qp′_(Cb)

ActQpOffset=PpsActQpOffsetCb   [Equation 83]

Alternatively, when the cIdx has a value of 2, the qP and the ACT Qpoffset may be derived as shown in the equation below.

qP=Qp′_(Cr)

ActQpOffset=PpsActQpOffsetCr   [Equation 84]

In addition, when the cIdx does not have a value of 0 and theTuCResMode[xTbY][yTbY] does not have a value of 0, the ACT Qp offset maybe derived as shown in the equation below.

ActQpOffset=(tu_cbf_cb[xTbY][yTbY])?PpsActQpOffsetCbCrModeA:PpsActQpOffsetCbCrModeB  [Equation 85]

In the meantime, in another embodiment, when the TuCResMode[xTbY][yTbY]has a value of 2, the ActQpOffset may be derived as shown in theequation below.

ActQpOffset=(tu_cbf_cb[xTbY][yTbY])?(PPsQpOffsetCbCrModeA+slice_act_CbCr_qp_offset_ModeA):(PPsQpOffsetCbCrModeB+slice_act_CbCr_qp_offset_ModeB)  [Equation 86]

In another embodiment of signaling an ACT Qp offset, only ACT QP offsetsfor Y, Cb, and Cr may be signaled as in the syntax table of FIG. 41. AnACT QP offset for joint CbCr may be derived from PpsActQpOffsetY,PpsActQpOffsetCb, and/or PpsActQpOffsetCr.

In an embodiment, an ACT Qp offset for CbCr may be set to a value ofPpsActQpOffsetCb. In another embodiment, an ACT Qp offset for CbCr maybe set to the same value as PpsActQpOffsetCb in the case of the jointCbCr mode in which the tu_cbf_cb has a non-zero value, or may be set tothe same value as PpsActQpOffsetCr in the case of the joint CbCr mode inwhich the tu_cbf_cb has a value of 0. Alternatively, it may be set viceversa.

FIG. 41 is a view showing another embodiment of a syntax table in whichan ACT Qp offset is signaled in a PPS. According to determination of thesyntax elements of FIG. 41, a quantization parameter qP may bedetermined as follows. First, when the cIdx has a value of 0, the qP andthe ACT Qp offset may be derived as shown in the equation below.

qP=Qp′_(Y)

ActQpOffset=PpsActQpOffsetY   [Equation 87]

Alternatively, when the TuCResMode[xTbY][yTbY] has a value of 2, the qPand the ACT Qp offset may be derived as shown in the equation below.

qP=Qp′_(CbCr)

ActQpOffset=(cIdx==1)?PpsActQpOffsetCb:PpsActQpOffsetCr   [Equation 88]

In the meantime, in another embodiment, a value of an ActQpOffset may bedetermined as follows.

ActQpOffset=(tu_cbf_cb[xTbY][yTbY])?PpsActQpOffsetCb:PpsActQpOffsetCr  [Equation 89]

Alternatively, when the cIdx has a value of 1, the qP and the ACT Qpoffset may be derived as shown in the equation below.

qP=Qp′_(Cb)

ActQpOffset=PpsActQpOffsetCb   [Equation 90]

Alternatively, when the cIdx has a value of 2, the qP and the ACT Qpoffset may be derived as shown in the equation below.

qP=Qp′_(Cr)

ActQpOffset=PpsActQpOffsetCr   [Equation 91]

Embodiment 7: Signaling of ACT Qp Offset at Multiple Levels

In an embodiment, an ACT QP offset may be signaled at a plurality oflevels. In addition to the signaling of an ACT QP offset at one level,such as a PPS, as in Embodiment 6 above, an ACT QP offset may besignaled at a lower level (e.g., a slice header, a picture header, orheaders of other types suitable for Qp control).

Hereinafter, two embodiments will be described. FIGS. 42 and 43 showexamples in which an ACT QP offset is signaled through a slice headerand a picture header. In this way, an ACT QP offset may be signaled atmultiple levels.

Hereinafter, the syntax elements shown in FIGS. 42 and 43 will bedescribed. A syntax element pps_slice_act_qp_offsets_present_flag mayindicate whether syntax elements slice_act_y_qp_offset,slice_act_cb_qp_offset, slice_act_cr_qp_offset, andslice_act_cbcr_qp_offset, which will be described later, are present ina slice header.

For example, a first value (e.g., 0) of thepps_slice_act_qp_offsets_present_flag may indicate that theslice_act_y_qp_offset, the slice_act_cb_qp_offset, theslice_act_cr_qp_offset, and the slice_act_cbcr_qp_offset are not presentin a slice header.

For example, a second value (e.g., 1) of thepps_slice_act_qp_offsets_present_flag may indicate that theslice_act_y_qp_offset, the slice_act_cb_qp_offset, theslice_act_cr_qp_offset, and the slice_act_cbcr_qp_offset are present ina slice header.

The syntax elements slice_act_y_qp_offset, slice_act_cb_qp_offset,slice_act_cr_qp_offset, and slice_act_cbcr_qp_offset may indicateoffsets for quantization parameter values qP for the luma, Cb, Crcomponents, and a joint CbCr component, respectively. The values of theslice_act_y_qp_offset, the slice_act_cb_qp_offset, theslice_act_cr_qp_offset, and the slice_act_cbcr_qp_offset may be limitedto have values ranging from −12 to 12. When the values of theslice_act_y_qp_offset, the slice_act_cb_qp_offset, theslice_act_cr_qp_offset, and the slice_act_cbcr_qp_offset are not presentin a bitstream, each value may be set to 0. Values ofPpsActQpOffsetY+slice_act_y_qp_offset,PpsActQpOffsetCb+slice_act_cb_qp_offset,PpsActQpOffsetCr+slice_act_cr_qp_offset, andPpsActQpOffsetCbCr+slice_act_cbcr_qp_offset may be limited to also havevalues ranging from −12 to 12.

Various modified embodiments of signaling an ACT QP offset for jointCbCr at a PPS level may be applied. For example, one QP offset may besignaled for joint CbCr, a plurality of ACT Qp offsets may be signaledfor joint CbCr of different modes, or without signaling an ACT Qp offsetfor joint CbCr, a method of deriving this by using ACTQpOffsets for Y,Cb, and Cr and/or a mode of jointCbCr may be applied when signalingthrough a slice header is performed.

Two modified embodiments are shown in FIGS. 44 and 45. FIG. 44 shows anembodiment in which an ACT Qp offset is signaled in a slice header. FIG.45 shows another embodiment in which an ACT Qp offset is signaled in aslice header. In FIG. 45, only ACT Qp offsets for Y, Cb, and Cr may besignaled, and an ACT QP offset at a slice level for joint CbCr may bederived from slice_act_y_qp_offset, slice_act_cb_qp_offset, and/orslice_act_cr_qp_offset. This may be determined on the basis of a modetype of jointCbCr. In an embodiment, a slice-level ACT Qp offset forCbCr may be set to the same value as the slice_act_cb_qp_offset. Inanother embodiment, in the case of the joint CbCr mode having thetu_cbf_cb having a non-zero value, an ACT Qp offset at a slice level forjoint CbCr may be set to the same value as the slice_act_cb_qp_offset.In addition, in the case of the jointCbCr mode having the tu_cbf_cbhaving a value of 0, an ACT Qp offset at a slice level for joint CbCrmay be set to the same value as the slice_act_cr_qp_offset.

In the meantime, in another embodiment, a syntax element may be signaledin a slice header or a picture header. To realize this,encoding/decoding may be performed as follows.

-   -   A flag pps_picture_slice_act_qp_offsets_present_flag indicating        whether an ACT Qp offset is present in a picture header or a        slice header may be signaled in a PPS.    -   When ACT is applicable and the value of the        pps_picture_slice_act_qp_offsets_present_flag is a second value        (e.g., 1), a flag pic_act_qp_offsets_present_flag indicating        whether an ACT Qp offset is present in a picture header is        signaled in a picture header. Herein, a second value (e.g., 1)        of the pic_act_qp_offsets_present_flag may indicate that ACT Qp        offsets for all slices of a picture corresponding to the picture        header are provided in the picture header.    -   A first value (e.g., 0) of the pic_act_qp_offsets_present_flag        may indicate that ACT Qp offsets for all slices of a picture        corresponding to the picture header are not provided in the        picture header. For example, when ACT is applicable and the        value of the pps_picture_slice_act_qp_offsets_present_flag is        the second value (e.g., 1) and the value of the        pic_act_qp_offsets_present_flag is the first value (e.g., 0), an        ACT Qp offset for a slice may be provided in a slice header.

FIG. 46 is a view showing a syntax table of a PPS in whichpps_pic_slice_act_qp_offsets_present_flag is signaled. The syntaxelement pps_pic_slice_act_qp_offsets_present_flag may indicate whetheran ACT Qp offset is provided in a picture header and/or a slice header.For example, a first value (e.g., 0) of thepps_pic_slice_act_qp_offsets_present_flag may indicate that an ACT Qpoffset is not provided in a picture header and a slice header. A secondvalue (e.g., 1) of the pps_pic_slice_act_qp_offsets_present_flag mayindicate that an ACT Qp offset is provided in a picture header or aslice header. When the pps_pic_slice_act_qp_offsets_present_flag is notprovided in a bitstream, the value of thepps_pic_slice_act_qp_offsets_present_flag may be determined to be thefirst value (e.g., 0).

FIG. 47 is a view showing a syntax table of a picture header forsignaling an ACT Qp offset. A syntax elementpic_act_qp_offsets_present_flag may indicate whether an ACT Qp offset isprovided in a picture header. A first value (e.g., 0) of thepic_act_qp_offsets_present_flag may indicate that an ACT Qp offset isnot provided in a picture header, but in a slice header. A second value(e.g., 1) of the pic_act_qp_offsets_present_flag may indicate that anACT Qp offset is provided in a picture header. When the value of thepic_act_qp_offsets_present_flag is not provided in a bitstream, thevalue may be determined to be 0.

FIG. 48 is a view showing a syntax table of a slice header for signalingan ACT Qp offset. In the syntax table of FIG. 48, syntax elementsslice_act_y_qp_offset, slice_act_cb_qp_offset, slice_act_cr_qp_offset,and slice_act_cbcr_qp_offset may indicate offsets for quantizationparameter values qP for luma, Cb, and Cr components. The values of theslice_act_y_qp_offset, the slice_act_cb_qp_offset, theslice_act_cr_qp_offset, and the slice_act_cbcr_qp_offset may have valuesranging from −12 to 12. In addition, values ofPpsActQpOffsetY+slice_act_y_qp_offset,PpsActQpOffsetCb+slice_act_cb_qp_offset, andPpsActQpOffsetCr+slice_act_cr_qp_offset may be limited to also have arange of values from −12 to 12.

In the meantime, in the case in which the values of theslice_act_y_qp_offset, the slice_act_cb_qp_offset, theslice_act_cr_qp_offset, and the slice_act_cbcr_qp_offset are notprovided in a bitstream, when the value of thepps_pic_slice_act_qp_offsets_present_flag is the first value (e.g., 0),the values of the slice_act_y_qp_offset, the slice_act_cb_qp_offset, andthe slice_act_cr_qp_offset may be determined to be 0. Alternatively,when the value of the pps_pic_slice_act_qp_offsets_present_flag is thesecond value (e.g., 1), the values of the slice_act_y_qp_offset, theslice_act_cb_qp_offset, and the slice_act_cr_qp_offset may be determinedto be the same values as pps_act_y_qp_offset, pps_act_cb_qp_offset, andpps_act_cr_qp_offset, respectively.

In the meantime, when an ACT Qp offset is present in both a slice headerand a picture header, a final offset value used to derive a qP value maybe determined to be the value that is obtained by adding an offset valuesignaled in a PPS and an offset value signaled in the slice header orthe picture header.

More specifically, in an embodiment, a quantization parameter qP may bedetermined as follows. First, when the cIdx has a value of 0, the qP andthe ACT Qp offset may be derived as shown in the equation below.

qP=Qp′_(Y)

ActQpOffset=PPsQpOffsetY+slice_act_y_qp_offset   [Equation 92]

Alternatively, when the TuCResMode[xTbY][yTbY] has a value of 2, the qPand the ACT Qp offset may be derived as shown in the equation below.

qP=Qp′_(CbCr)

ActQpOffset=PPsQpOffsetCbCr+slice_act_CbCr_qp_offset   [Equation 93]

Alternatively, when the cIdx has a value of 1, the qP and the ACT Qpoffset may be derived as shown in the equation below.

qP=Qp′_(Cb)

ActQpOffset=PpsActQpOffsetCb+slice_act_Cb_qp_offset   [Equation 94]

Alternatively, when the cIdx has a value of 2, the qP and the ACT Qpoffset may be derived as shown in the equation below.

qP=Qp′_(Cr)

ActQpOffset=PpsActQpOffsetCr+slice_act_Cr_qp_offset   [Equation 95]

In another embodiment, when multiple ACT Qp offsets for joint CbCr aresignaled, ActQpOffset for joint CbCr may be determined as follows.

First, when the cIdx has a value of 0, the qP and the ACT Qp offset maybe derived as shown in the equation below.

qP=Qp′_(Y)

ActQpOffset=PPsQpOffsetY+slice_act_y_qp_offset   [Equation 96]

Alternatively, when the TuCResMode[xTbY][yTbY] has a value of 2, the qPmay be derived as shown in the equation below.

qP=Qp′_(CbCr)   [Equation 97]

Alternatively, when the cIdx has a value of 1, the qP and the ACT Qpoffset may be derived as shown in the equation below.

qP=Qp′_(Cb)

ActQpOffset=PpsActQpOffsetCb+slice_act_Cb_qp_offset   [Equation 98]

Alternatively, when the cIdx has a value of 2, the qP and the ACT Qpoffset may be derived as shown in the equation below.

qP=Qp′_(Cr)

ActQpOffset=PpsActQpOffsetCr+slice_act_Cr_qp_offset   [Equation 99]

In addition, when the cIdx does not have a value of 0 and theTuCResMode[xTbY][yTbY] does not have a value of 0, the ACT Qp offset maybe derived as shown in the equation below.

ActQpOffset=(tu_cbf_cb[xTbY][yTbY])?(PPsQpOffsetCbCrModeA+slice_act_CbCr_qp_offset_ModeA):(PPsQpOffsetCbCrModeB+slice_act_CbCr_qp_offset_ModeB)  [Equation 100]

In still another embodiment, when an ACT Qp offset for joint CbCr is notprovided, qP and ActQpOffset for Y, Cb, and/or Cr components aredetermined and the ActQpOffset for joint CbCr may be determined usingACT Qp offsets of the Y, Cb, and/or Cr components as follows. Forexample, in the above-described embodiment, when theTuCResMode[xTbY][yTbY] related to Equation 97 has a value of 2, thecalculation step of the qP may be changed and performed as follows.

“Alternatively, when the TuCResMode[xTbY][yTbY] has a value of 2, the qPand the ACT Qp offset may be derived as shown in the equation below.

qP=Qp′_(CbCr)

ActQpOffset=(cIdx==1)?(PPsQpOffsetCb+slice_act_Cb_qp_offset):(PPsQpOffsetCr+slice_act_Cr_qp_offset)”  [Equation101]

In the meantime, in another embodiment, a value of ActQpOffset may bedetermined as in the equation below.

ActQpOffset=(tu_cbf_cb[xTbY][yTbY])?(PPsQpOffsetCb+slice_act_Cb_qp_offset):(PPsQpOffsetCr+slice_act_Cr_qp_offset)  [Equation 102]

Embodiment 8: Method of Signaling Multiple ACT Qp Offset Sets

In the present embodiment, a method of using a list of ACT Qp offsetswill be described. To this end, the following processing may beperformed.

a) Multiple sets of ACT Qp offsets may be signaled in the form of a listwithin a parameter set (e.g., SPS or PPS). Each set in the list mayinclude ACT Qp offsets for Y, Cb, Cr, and joint CbCr components. Forsimplicity, the list of ACT Qp offsets may be signaled in the parameterset the same as the parameter set for signaling a list of chroma Qpoffsets.

b) The number of the sets of the ACT Qp offsets in the list may be thesame as the number of chroma Qp offset sets signaled in a PPS.

c) As an ACT Qp offset used to derive the qP for each encoding basis, anACT Qp offset belonging to the list having an index (e.g.,cu_chroma_qp_offset_idx) for a chroma Qp offset for the encoding basismay be used.

d) As an alternative embodiment of b) and c), the following may beperformed.

-   -   The number of the sets of the ACT Qp offsets in the list may be        signaled. The number of the sets of the ACT Qp offsets in the        list may be different from the number of chroma Qp offset sets.    -   When ACT is applicable, an index indicating the index of the ACT        Qp offset used for the encoding basis may be signaled.

Without departing from the above concept, the syntax for signaling alist of ACT Qp offsets may be used as shown in FIG. 49. For example,when cu_act_enabled_flag has a value of 1, pps_act_y_qp_offset,pps_act_cb_qp_offset, pps_act_cr_qp_offset, and pps_act_cbcr_qp_offsetmay be used to determine offsets to be applied to quantization parametervalues qP for luma, Cb, Cr components, and joint CbCr, respectively.

When the values of the pps_act_y_qp_offset, the pps_act_cb_qp_offset,the pps_act_cr_qp_offset, and the pps_act_cbcr_qp_offset are notpresent, each value may be derived into 0.

When the value of the cu_act_enabled_flag is a second value (e.g., 1)and a value of cu_chroma_qp_offset_flag is a second value (e.g., 1),act_y_qp_offset_list[i], act_cb_qp_offset_list[i],act_cr_qp_offset_list[i], and act_cbcr_qp_offset_list[i] may be used todetermine offsets applied to quantization parameters value qP for luma,Cb, Cr components, and a joint CbCr component, respectively. When thevalues of the act_y_qp_offset_list[i], the act_cb_qp_offset_list[i], theact_cr_qp_offset_list[i], and the act_cbcr_qp_offset_list[i] are notpresent, each value may be derived into 0.

In the present embodiment, a quantization parameter qP may be determinedas follows. First, when the cIdx has a value of 0, the qP and the ACT Qpoffset may be derived as shown in the equation below.

qP=Qp′_(Y)   [Equation 103]

ActQpOffset=pps_act_y_qp_offset+(cu_chroma_qp_offset_flag)?act_y_qp_offset_list[cu_chroma_qp_offset_idx]:0+slice_act_y_qp_offsetAlternatively, when the TuCResMode[xTbY][yTbY] has a value of 2, the qPand the ACT Qp offset may be derived as shown in the equation below.

qP=Qp′_(CbCr)

ActQpOffset=pps_act_cbcr_qp_offset+(cu_chroma_qp_offset_flag)?act_cbcr_qp_offset_list[cu_chroma_qp_offset_idx]:0+slice_act_cbcr_qp_offset  [Equation 104]

Alternatively, when the cIdx has a value of 1, the qP and the ACT Qpoffset may be derived as shown in the equation below.

qP=Qp′_(Cb)

ActQpOffset=pps_act_cb_qp_offset+(cu_chroma_qp_offset_flag)?act_cb_qp_offset_list[cu_chroma_qp_offset_idx]:0+slice_act_cb_qp_offset  [Equation 105]

Alternatively, when the cIdx has a value of 2, the qP and the ACT Qpoffset may be derived as shown in the equation below.

qP=Qp′_(Cr)

ActQpOffset=pps_act_cr_qp_offset+(cu_chroma_qp_offset_flag)?act_cr_qp_offset_list[cu_chroma_qp_offset_idx]:0+slice_act_cr_qp_offset  [Equation 106]

Embodiment 9: ACT Color Space Conversion Method to be Applied to BothLossless Encoding and Lossy Encoding

Conversion between color spaces based on the matrices for forwardconversion and backward conversion described above may be organized asfollows.

TABLE 3 Forward Conversion: GBR to YCgCo Backward Conversion: YCgCo toGBR Y = ((G<<1) + R + B + 2) >>.2 t = Y − Cg Cg = ((G<<1) − R − B +2) >>2 G = Y + Cg Co = ((R−B) << 1) + 2) >> 2 B = t − Co = Y − Cg − Co R= t + Co = Y − Cg + Co

The conversion is unable to achieve reconstruction into the originalstate because loss of some values occurs in the processing of Co and Cg.For example, when a sample value of the RGB color space is convertedinto that of the YCgCocolor space and the resulting value is inverselyconverted back to that of the RGB color space, the value of the originalsample is not completely reconstructed. Therefore, the conversionaccording to Table 3 cannot be used for lossless encoding. It isnecessary to improve a color space conversion algorithm so that loss ofa sample value does not occur after color space conversion even whenlossless encoding is applied. Embodiments 9 and 10 describes a colorspace conversion algorithm capable of being applied to lossless encodingas well as lossy encoding.

In the embodiment described below, described is a method of performingACT by using color space conversion reversible into an original state,which is applicable to lossy encoding as well as lossless encoding. Thisreversible color space conversion may be applied to the above-describedencoding and decoding methods. An ACT Qp offset may also be adjusted forthe color space conversion below. Color space conversion according to anembodiment may be performed as shown in the equation below. For example,forward conversion from a GBR color space to a YCgCo color space may beperformed according to the equation below.

Co=R−B;

t=B+(Co>>1);

Cg=G−t;

Y=t+(Cg>>1);   [Equation 107]

In addition, backward conversion from a YCgCo color space to a GBR colorspace may be performed according to the equation below.

t=Y−(Cg>>1)

G=Cg+t

B=t−(Co>>1)

R=Co+B

The conversion between the YCgCo color space and the RGB color spaceaccording to the equations above is reversible to an original state.That is, the color space conversion according to the equations supportsperfect reconstruction. For example, a sample value is maintained eventhough backward conversion is performed after forward conversion.Accordingly, the color space conversion according to the equations maybe referred to as reversible YCgCo-R color conversion. Herein, R maystand for reversible, which means that achieves reconstruction to anoriginal state. YCgCo-R conversion may be provided by increasing the bitdepths of Cg and Co by 1 compared to the existing conversion. If thiscondition is met, other types of reversible conversion may be used likethe above-described conversion.

Because the conversion as shown in the equations above has a differentnorm value from the above-described conversion, ACT Qp offsets for Y,Cg, and Co may be adjusted to compensate for a dynamic range change dueto color space conversion.

It has been described that when the above-described conversion isapplied, a QCT Qp offset according to an embodiment may have a value of(−5, −5, −5) for Y, Cg, and Co. However, when the reversible conversionaccording to the present embodiment is applied, as a QCT Qp offsetaccording to an embodiment, another value not (−5, −5, −5) may bespecified. For example, as a QCT Qp offset according to an embodiment, avalue of (−5, 1, 3) for Y, Cg, and Co may be used.

In another embodiment, an ACT QP offset may be signaled through abitstream as in Embodiment 6 or 7 described above.

For example, when the YCgCo-R conversion described above was usedtogether with an ACT QP offset of (−5, 1, 3), it was observed thatencoding loss was not present in a lossy encoding environment (e.g., QP22, 27, 32, 37), as shown in the figure below. In addition, it wasobserved that when ACT is applied, encoding performance of 5% wasfurther obtained in realizing lossless encoding.

TABLE 4 Sequence Y U V RGB, TGM 1080p 0.0% 0.2% 0.1% RGB, TGM 720p 0.2%−0.1% 0.1% RGB, Animation −0.1% −0.1% 0.0% RGB, Mixed content −0.1% 0.0%−0.1% RGB, Camera-Captured content −0.3% 0.2% −0.3% Overall All (RGB)0.0% 0.0% 0.0%

The VVC specification for including an integrated ACT matrix may bedescribed as the table below.

TABLE 5 Residual modification process for blocks using colour spaceconversion Inputs to this process are: - a variable nTbW specifying theblock width, - a variable nTbH specifying the block height, - an(nTbW)x(nTbH) array of luma residual samples r_(Y) with elements r_(Y)[x ][ y ], - an (nTbW)x(nTbH) array of chroma residual samples r_(Cb)with elements r_(Cb)[ x ][ y ], - an (nTbW)x(nTbH) array of chromaresidual samples r_(Cr) with elements r_(Cr)[ x ][ y ]. Outputs of thisprocess are: - a modified (nTbW)x(nTbH) array r_(Y) of luma residualsamples, - a modified (nTbW)x(nTbH) array r_(Cb) of chroma residualsamples, - a modified (nTbW)x(nTbH) array r_(Cr) of chroma residualsamples. The (nTbW)x(nTbH) arrays of residual samples r_(Y), r_(Cb) andr_(Cr) are modified as follows: tmp = r_(Y)[ x ][ y ] − (r_(Cb)[ x ][ y] >>1) r_(Y)[ x ][ y ] = tmp + r_(Cb)[ x ][ y ] r_(Cb)[ x ][ y ] = tmp −(r_(Cr)[ x ][ y ] >>1) r_(Cr)[ x ][ y ] = r_(Cb)[ x ][ y ] + r_(Cr)[ x][ y ]

For example, (nTbW)×(nTbH)-sized residual sample arrays r_(Y), r_(Cb),and r_(Cr) may be updated as follows.

tmp=r _(Y)[x][y]−(r _(Cb)[x][y]>>1)

r _(Y)[x][y]=tmp+r _(Cb)[x][y]

r _(Cb)[x][y]=tmp−(r _(Cr)[x][y]>>1)

r _(Cr)[x][y]=r _(Cb)[x][y]+r _(Cr)[x][y]  [Equation 109]

Embodiment 10: ACT Performance Method for Performing Multiple ColorConversions Based on Explicit Signaling

In the present embodiment, at least one color conversion may beperformed by ACT. Which color conversion is to be performed may bedetermined by a flag(s) signaled in a bitstream. The flag(s) may besignaled at multiple levels, such as an SPS, a PPS, a picture header,and a slice, or identifiable granularities.

In an embodiment, a predetermined flag may be signaled to indicate whichACT is applied. For example, when the flag has a value of 1, ACT basedon reversible color conversion may be applied. When the flag has a valueof 0, ACT based on non-reversible color conversion may be applied.

In another embodiment, a predetermined flag for ACT may be signaled toindicate which color conversion is used. An example of syntax signaledin an SPS is shown in FIG. 50. The syntax element of FIG. 50 will bedescribed. A syntax element sps_act_reversible_conversion may indicatewhether a conversion equation unreversible to an original state is used.A first value (e.g., 0) of the sps_act_reversible_conversion mayindicate that ACT uses a conversion equation unreversible to an originalstate. A second value (e.g., 1) of the sps_act_reversible_conversion mayindicate that ACT uses a conversion equation reversible to an originalstate.

Accordingly, a variable lossyCoding indicating whether lossy encoding isperformed may be set as in the equation below.

lossyCoding=(!sps_act_reversible_conversion)   [Equation 110]

By using the lossyCoding flag, the pseudocode for the decoding device toperform backward conversion from YCgCo to GBR in the decoding processmay be expressed as follows.

  [Equation 111] If (sps_act_reversible_conversion == 1) { // YCgCo-Rreversible conversion t = Y − (Cg>>1) G = Cg + t B = t − (Co>>1) R =Co + B } else { t = Y − Cg G = Y + Cg B = t − Co R = t + Co }

Accordingly, the VVC specification shown in Table 9 of Embodiment 9 maybe modified as shown in the table below.

TABLE 6 Residual modification process for blocks using colour spaceconversion Inputs to this process are: - a variable nTbW specifying theblock width, - a variable nTbH specifying the block height, - an(nTbW)x(nTbH) array of luma residual samples r_(Y) with elements r_(Y)[x ][ y ], - an (nTbW)x(nTbH) array of chroma residual samples r_(Cb)with elements r_(Cb)[ x ][ y ], - an (nTbW)x(nTbH) array of chromaresidual samples r_(Cr) with elements r_(Cr)[ x ][ y ]. Outputs of thisprocess are: - a modified (nTbW)x(nTbH) array r_(Y) of luma residualsamples, - a modified (nTbW)x(nTbH) array r_(Cb) of chroma residualsamples, - a modified (nTbW)x(nTbH) array r_(Cr) of chroma residualsamples. The (nTbW)x(nTbH) arrays of residual samples r_(Y), r_(Cb) andr_(Cr) are modified as follows:  - If sps_act_ reversible _conversionequal to 1, the (nTbW)x(nTbH) arrays of    residual samples r_(Y),r_(Cb) and r_(Cr) are modified as follows   tmp = r_(Y)[ x ][ y ] −(r_(Cb)[ x ][ y ] >>1) )   r_(Y)[ x ][ y ] = tmp + r_(Cb)[ x ][ y ] )  r_(Cb)[ x ][ y ] = tmp − (r_(Cr)[ x ][ y ] >>1) )    r_(Cr)[ x ][ y ]= r_(Cb)[ x ][ y ] + r_(Cr)[ x ][ y ]  - Otherwise, the (nTbW)x(nTbH)arrays of residual samples r_(Y), r_(Cb) and r_(Cr) are    modified asfollows   tmp = r_(Y)[ x ][ y ] − r_(Cb)[ x ][ y ] )   r_(Y)[ x ][ y ] =r_(Y)[ x ][ y ] + r_(Cb)[ x ][ y ] )   r_(Cb)[ x ][ y ] = tmp − r_(Cr)[x ][ y ] )    r_(Cr)[ x ][ y ] = tmp + r_(Cr)[ x ][ y ]

According to the table above, a residual update process using colorspace conversion may use the following parameters as inputs to thepresent process.

-   -   a variable nTbW indicating a block width    -   a variable nTbH indicating a block height    -   a (nTbW)×(nTbH)-sized array r_(Y) for a luma residual sample,        composed of elements r_(Y)[x][y],    -   a (nTbW)×(nTbH)-sized array r_(Cb) for a chroma residual sample,        composed of elements r_(Cb)[x][y],    -   a (nTbW)×(nTbH)-sized array r_(Cr) for a chroma residual sample,        composed of elements r_(Cr)[x][y],

The output for the present process is as follows.

-   -   a (nTbW)×(nTbH)-sized updated array r_(Y) for a luma residual        sample,    -   a (nTbW)×(nTbH)-sized updated array r_(Cb) for a chroma residual        sample,    -   a (nTbW)×(nTbH)-sized updated array r_(Cr) for a chroma residual        sample,

By performing the present process, (nTbW)×(nTbH)-sized residual samplearrays r_(Y), r_(Cb), and r_(Cr) may be updated as follows.

First, when the value of the sps_act_reversible_conversion is the secondvalue (e.g., 1), (nTbW)×(nTbH)-sized residual sample arrays r_(Y),r_(Cb), and r_(Cr) may be updated as shown in the equation below.

tmp=r _(Y)[x][y]−(r _(Cb)[x][y]>>1))

r _(Y)[x][y]=tmp+r _(Cb)[x][y])

r _(Cb)[x][y]=tmp−(r _(Cr)[x][y]>>1))

r _(Cr)[x][y]=r _(Cb)[x][y]+r _(Cr)[x][y]  [Equation 112]

Otherwise (e.g., when the value of the sps_act_reversible_conversion isthe first value (e.g., 0)), (nTbW)×(nTbH)-sized residual sample arraysr_(Y), r_(Cb), and r_(Cr) may be updated as shown in the equation below.

tmp=r _(Y)[x][y]−r _(Cb)[x][y]

r _(Y)[x][y]=r _(Y)[x][y]+r _(Cb)[x][y]

r _(Cb)[x][y]=tmp−r _(Cr)[x][y]

r _(Cr)[x][y]=tmp+r _(Cr)[x][y]  [Equation 113]

YCgCo backward conversion and YCgCo-R backward conversion have somesimilarities. In conversion reversible to an original state, when Cg andCo are replaced with Cg′=Cg<<1 and Co′=Co<<1, this may operate as lossybackward conversion. The equation below shows an embodiment thereof.

t=Y−(Cg′>>1)=Y−Cg

G=Cg′+t=Y+Cg

B=t−(Co′>>1)=t−Co=Y−Cg−Co

R=Co′+B=t+Co=Y−Cg+Co   [Equation 114]

Accordingly, in an alternative embodiment, instead of maintaining twocolor conversions, only conversion reversible to an original state maybe used. In the lossy encoding case, Cg and Co components may be scaled1/2 times in the operation of the encoding device, and may be scaledtwice in the operation of the decoding device. This enables oneintegrated conversion to be used even when lossy and lossless cases aresupported. In addition, there is an additional advantage that bit depthdoes not change even when lossy encoding is in progress.

TABLE 7 Backward Conversion : YCgCo to GBR If (lossyCoding) {  Cg = Cg<< 1;  Co = Co << 1; } t = Y − (Cg>>1) G = Cg + t B = t − (Co>>1) R =Co + B

In an embodiment, a flag (e.g., actShiftFlag) indicating which ACTconversion is used may be used according to the syntax of FIG. 51. Inthe syntax table of FIG. 51, a syntax element sps_act_shift_flag mayindicate whether performing color component shifting is applied whileACT is applied. For example, a first value (e.g., 0) of thesps_act_shift_flag may indicate that performing color component shiftingis not applied while ACT is applied. A second value (e.g., 1) of thesps_act_shift_flag may indicate that performing color component shiftingis applied while ACT is applied. The variable actShiftFlag may be set tothe value of the sps_act_shift_flag. The pseudocode for realizingbackward conversion from YCgCo to GBR in the decoding device may bewritten using the actShiftFlag, as follows.

TABLE 8 Backward Conversion: YCgCo to GBR If (actShiftFlag) {  Cg = Cg<< 1;  Co = Co << 1; } t = Y − (Cg>>1) G = Cg + t B = t − (Co>>1) R =Co + B

Embodiment 11: ACT Performance Method for Performing Multiple ColorConversions Using Derivation of Transform Type

-   In an embodiment, at least one color conversion may be used in    performing ACT. In addition, which color conversion type is used may    be derived on the basis of other information of a bitstream.

In an embodiment, two types of ACT conversions may be availableincluding ACT conversion reversible to an original state and ACTconversion unreversible to an original state. The ACT conversion typesmay be derived by transform types. For example, as identified by avariable tuIsTransformSkip, when the transform type is transform skip,the ACT conversion reversible to an original state may be used.Otherwise (e.g., when the transform type is not transform skip), the ACTconversion unreversible to an original state may be used. Two types ofpseudocode may be used.

TABLE 9 Backward Conversion: YCgCo to GBR If (tuIsTransformSkip) {  //YCgCo-R reversible conversion   t = Y − (Cg>>1)   G = Cg + t   B = t −(Co>>1)   R = Co + B } else {   t = Y − Cg  G = Y + Cg  B = t − Co  R =t + Co }

TABLE 10 Backward Conversion: YCgCo to GBR If (tuIsTransformSkip) { Cg = Cg << 1;  Co = Co << 1; } t = Y − (Cg>>1) G = Cg + t B = t −(Co>>1) R = Co + B

In another embodiment, the ACT conversion types may be determined on thebasis of QP values. When a Qp value is less than or equal to apredetermined threshold value (e.g., QpPrimeTsMin), the ACT conversionreversible to an original state may be used. Otherwise (e.g., when theQp value exceeds the predetermined threshold value), the non-reversibleACT conversion may be used.

Embodiment 12: OP Derivation Method Using ACT OP Offset

The present embodiment is related to Embodiments 1 and 2 describedabove. In Embodiments 1 and 2, it has already been described thatderived Qp′_(Y), Qp′_(CbCr), Qp′_(Cb), and Qp′_(Cr) are included as QPs.The methods described in Embodiments 1 and 2 correct the derived Qpvalues by using an ACT QP offset, and apply an essential clippingtechnique so that the corrected QP values for transform coefficientscaling are not out of an effective range.

The present embodiment describes a method of including an ACT QP offsetin a QP derivation process for deriving Qp′_(Y), Qp′_(CbCr), Qp′_(Cb),and Qp′_(Cr). The QP derivation process already includes a predeterminedclipping step so that the derived QP values are not out of the effectiverange. Therefore, including an ACT QP offset in the QP derivationprocess may avoid additional clipping step, simplify the overall QPderivation step for transform coefficient scaling process, and ensurethat the final QP is not out of the effective range.

As described in the embodiment described above, an ACT QP offset may bepre-specified to a constant, or may be signaled through a bitstream.Without departing from consistency, ACT QP offsets for Y, Cb, Cr, andCbCr may be described as ppsActQpOffsetY, ppsActQpOffsetCb,ppsActQpOffsetCr, and ppsActQpOffsetCbCr in the description to bedescribed later. The ppsActQpOffsetY, the ppsActQpOffsetCb, theppsActQpOffsetCr, and the ppsActQpOffsetCbCr may be constants orvariables having values ranging from −M to N. Herein, each of the M andN may be set to 12 in the lossy encoding case and may be set to 0 in thelossless encoding case in an embodiment. In addition, at least one ACTQP offset may be derived from another ACT QP offset value. For example,the ppsActQpOffsetCbCr may be set to the same value as theppsActQpOffsetCb or the ppsActQpOffsetCr, on the basis of the jointCbCrmode.

Decoding processing for QP derivation using an ACT QP offset may beperformed as described below. First, in the case of a quantizationparameter derivation process, the following parameters may be utilizedfor the present process.

-   -   Luma coordinates (xCb, yCb) indicating relative coordinates of        the top left luma sample of a current encoding block with        respect to the top left luma sample of a current picture,    -   A variable cbWidth indicating the width of the current encoding        block on a per luma sample basis,    -   A variable cbHeight indicating the height of the current        encoding block on a per luma sample basis    -   A variable treeType indicating whether a single tree        (SINGLE_TREE) or a dual tree is used to partition a current        coding tree node, and indicating, when the dual tree is used,        whether the dual tree is a luma component dual tree        (DAUL_TREE_LUMA) or a chroma component dual tree        (DAUL_TREE_CHROMA)

In the present quantization parameter derivation process, a lumaquantization parameter Qp′_(Y) and chroma quantization parametersQp′_(Cb), Qp′_(Cr), and Qp′_(CbCr) may be derived.

Afterward, a variable Qp_(Y) may be derived as shown in the equationbelow.

Qp_(Y)=((qP_(Y_PRED)+CuQpDeltaVal+64+2*QpBdOffset)%(64+QpBdOffset))−QpBdOffset  [Equation 115]

The luma quantization parameter Qp′_(Y) may be derived as shown in theequation below.

actQpOffsetY=cu_act_enabled_flag[xCb][yCb]?ppsActQpOffsetY:0

Qp′_(Y)=Qp_(Y)+QpBdOffset+actQpOffsetY

Qp′_(Y)=Clip3(0, 63+QpBdOffset, Qp′_(Y))   [Equation 116]

When the value of a variable ChromaArrayType indicating a type of achroma array is not a first value (e.g., 0) and the treeType is theSINGLE_TREE or the DUAL_TREE_CHROMA, the following processing may beperformed.

-   -   When the value of the treeType is the DUAL_TREE_CHROMA, the        value of the variable Qp_(Y) may be set to the same value as        that of the luma quantization parameter Qp_(Y) of the luma        encoding basis covering the luma sample location (xCb+cbWidth/2,        yCb+cbHeight/2).    -   Variables qP_(Cb), qP_(Cr), and qP_(CbCr) may be derived as        shown in the equation below.

[Equation 117]

qP_(Chroma)=Clip3(−QpBdOffset, 63, Qp_(Y))   (1118)

qP_(Cb)=ChromaQpTable[0][qP_(Chroma)]  (1119)

qP_(Cr)=ChromaQpTable[1][qP_(Chroma)]  (1120)

qP_(CbCr)=ChromaQpTable[2][qP_(Chroma)]  (1121)

-   -   The chroma quantization parameters Qp′_(Cb) and Qp′_(Cr) for Cb        and Cr components and the chroma quantization parameter        Qp′_(CbCr) for joint Cb-Cr coding may be derived as shown in the        equation below.

actQpOffsetCb=cu_act_enabled_flag[xCb][yCb]?ppsActQpOffsetCb:0

actQpOffsetCr=cu_act_enabled_flag[xCb][yCb]?ppsActQpOffsetCr:0

actQpOffsetCbCr=cu_act_enabled_flag[xCb][yCb]?ppsActQpOffsetCbCr:0

Qp′_(Cb=Clip)3(−QpBdOffset, 63,qP_(Cb)+pps_cb_qp_offset+slice_cb_qp_offset+CuQpOffset_(Cb)+actQpOffsetCb)+QpBdOffset

Qp′_(Cr)=Clip3(−QpBdOffset, 63,qP_(Cr)+pps_cr_qp_offset+slice_cr_qp_offset+CuQpOffset_(Cr)+actQpOffsetCr)+QpBdOffset

Qp′_(CbCr)=Clip3(−QpBdOffset, 63,qP_(CbCr)+pps_joint_cbcr_qp_offset+slice_joint_cbcr_qp_offset+CuQpOffset_(CbCr)+actQpOffsetCbCr)+QpBdOffset  [Equation 118]

Next, a dequantization process for a transform coefficient may beperformed, and the following information may be utilized as an input forthe present process.

-   -   Luma coordinates (xTbY, yTbY) referring to relative coordinates        of the top left sample of the current luma transformation block        with respect to the top left luma sample of the current picture,    -   A variable nTbW indicating the width of a transformation block,    -   A variable nTbH indicating the height of a transformation block,    -   A variable predMode indicating a prediction mode of the encoding        basis,    -   A variable cIdx indicating a color component of a current block

The output of the dequantization process for the transform coefficientmay be an array d of scaled transform coefficients. Herein, the size ofthe array d may be (nTbW)×(nTbH). The individual elements constitutingthis may be identified as d[x][y].

In performing the present process, a quantization parameter qP may bederived as follows. When the cIdx has a value of 0, the qP may bederived as shown in the equation below.

qP=Qp′_(Y)   [Equation 119]

Alternatively, when the TuCResMode[xTbY][yTbY] has a value of 2, the qPmay be derived as shown in the equation below.

qP=Qp′_(CbCr)   [Equation 120]

Alternatively, when the cIdx has a value of 1, the qP may be derived asshown in the equation below.

qP=Qp′_(Cb)   [Equation 121]

Alternatively, when the cIdx has a value of 2, the qP may be derived asshown in the equation below.

qP=Qp′_(Cr)   [Equation 122]

The quantization parameter qP may be updated as follows. In addition,variables rectNonTsFlag and bdShift may be derived as follows. When thetransform_skip_flag[xTbY][yTbY][cIdx] has a value of 0, the variablesmay be derived as shown in the equation below.

rectNonTsFlag=(((Log2(nTbW)+Log2(nTbH))& 1)==1)?1:0

bdShift=BitDepth+rectNonTsFlag+((Log2(nTbW)+Log2(nTbH))/2)−5+pic_dep_quant_enabled_flag  [Equation 123]

Alternatively, when the transform_skip_flag[xTbY][yTbY][cIdx] has avalue of 1 (e.g., transform of the current trasnform block is skipped),the parameter and the variables may be derived as shown in the equationbelow.

qP=Max(QpPrimeTsMin, qP)

rectNonTsFlag=0

bdShift=10   [Equation 124]

Encoding and Decoding Methods

Hereinafter, an image encoding method performed by an image encodingdevice and an image decoding method performed by an image decodingdevice will be described with reference to FIGS. 52 and 53.

First, the operation of the decoding device will be described. The imagedecoding device according to an embodiment may include a memory and aprocessor. The decoding device may perform decoding according to theoperation of the processor. For example, as shown in FIG. 52, thedecoding device may determine a quantization parameter of a currentblock on the basis of whether color space conversion is applied to aresidual sample of the current block in step S5210. Herein, the colorspace conversion may be the above-described ACT. Herein, the determiningof the quantization parameter may be performed by performing clipping onthe quantization parameter such that a value of the quantizationparameter has a value less than or equal to a predetermined upper limitvalue and greater than or equal to a predetermined lower limit value.Herein, the predetermined lower limit value of the quantizationparameter may be 0. In addition, the predetermined upper limit value ofthe quantization parameter may be determined on the basis of a syntaxelement indicating a bit depth of a sample.

The determining of the quantization parameter may include: determiningthe quantization parameter on the basis of a color component of thecurrent block; determining a quantization parameter offset on the basisof the color component of the current block; and resetting thequantization parameter by using the quantization parameter offset.

The resetting of the quantization parameter by using the quantizationparameter offset may be performed by adding the quantization parameteroffset to the quantization parameter. Herein, when the color spaceconversion is applied to the residual sample of the current block andthe color component of the current block is a luma component, a value ofthe quantization parameter offset may be determined to be −5.

In addition, when the color space conversion is applied to the residualsample of the current block and the color component of the current blockis a chroma Cb component, a value of the quantization parameter offsetmay be determined to be 1. When the color space conversion is applied tothe residual sample of the current block and the color component of thecurrent block is a chroma Cr component, a value of the quantizationparameter offset may be determined to be 3.

Next, the decoding device may determine a transform coefficient of thecurrent block on the basis of the quantization parameter in step S5220.Next, the decoding device may determine the residual sample of thecurrent block by using the transform coefficient in step S5230. Next,the decoding device may reset a value of the residual sample on thebasis of whether the color space conversion is applied, in step S5240.

Herein, the resetting of the value of the residual sample may beperformed on the basis of a luma component residual sample value and ahalf value of a chroma residual sample value. For example, the halfvalue of the chroma residual sample value may be obtained by performingshift operation on the chroma residual sample value. In addition, a halfvalue of a chroma Cb component residual sample value may be added to theluma component residual sample value to reset the luma componentresidual sample value. In addition, a half value of a chroma Cbcomponent residual sample value and a half value of a chroma Crcomponent residual sample value are subtracted from the luma componentresidual sample value to reset the chroma Cb component residual samplevalue.

Hereinafter, the operation of the encoding device will be described. Theimage encoding device according to an embodiment may include a memoryand a processor. The encoding device may perform encoding according tothe operation of the processor in a manner that corresponds to thedecoding by the decoding device. For example, as shown in FIG. 53, theencoding device may reset a residual sample on the basis of whethercolor space conversion is applied in step S5310. Herein, the color spaceconversion may be the above-described ACT.

Further, the resetting of a value of the residual sample may beperformed on the basis of a luma component residual sample value and ahalf value of a chroma residual sample value. For example, the halfvalue of the chroma residual sample value may be obtained by performingshift operation on the chroma residual sample value. In addition, byperforming the resetting operation by the decoding device describedabove in reverse, the encoding device may reset a luma componentresidual sample value, a chroma Cb component residual sample value, anda chroma Cr component residual sample value.

Next, the encoding device may determine a transform coefficient by usingthe reset residual sample in step S5320. Next, the encoding device maydetermine a quantization parameter on the basis of whether the colorspace conversion is applied, in step S5330. Herein, the determining ofthe quantization parameter may be performed by performing clipping onthe quantization parameter such that a value of the quantizationparameter has a value less than or equal to a predetermined upper limitvalue and greater than or equal to a predetermined lower limit value.Herein, the predetermined lower limit value of the quantizationparameter may be 0. In addition, the predetermined upper limit value ofthe quantization parameter may be determined on the basis of a syntaxelement indicating a bit depth of a sample.

The determining of the quantization parameter may include: determiningthe quantization parameter on the basis of a color component of thecurrent block; determining a quantization parameter offset on the basisof the color component of the current block; and resetting thequantization parameter by using the quantization parameter offset.

The resetting of the quantization parameter by using the quantizationparameter offset may be performed by adding the quantization parameteroffset to the quantization parameter. Herein, when the color spaceconversion is applied to the residual sample of the current block andthe color component of the current block is a luma component, a value ofthe quantization parameter offset may be determined to be −5.

In addition, when the color space conversion is applied to the residualsample of the current block and the color component of the current blockis a chroma Cb component, a value of the quantization parameter offsetmay be determined to be 1. When the color space conversion is applied tothe residual sample of the current block and the color component of thecurrent block is a chroma Cr component, a value of the quantizationparameter offset may be determined to be 3.

Next, the encoding device may encode the transform coefficient on thebasis of the quantization parameter in step S5340.

Application Embodiment

While the exemplary methods of the present disclosure described aboveare represented as a series of operations for clarity of description, itis not intended to limit the order in which the steps are performed, andthe steps may be performed simultaneously or in different order asnecessary. In order to implement the method according to the presentdisclosure, the described steps may further include other steps, mayinclude remaining steps except for some of the steps, or may includeother additional steps except for some steps.

In the present disclosure, the image encoding apparatus or the imagedecoding apparatus that performs a predetermined operation (step) mayperform an operation (step) of confirming an execution condition orsituation of the corresponding operation (step). For example, if it isdescribed that predetermined operation is performed when a predeterminedcondition is satisfied, the image encoding apparatus or the imagedecoding apparatus may perform the predetermined operation afterdetermining whether the predetermined condition is satisfied.

The various embodiments of the present disclosure are not a list of allpossible combinations and are intended to describe representativeaspects of the present disclosure, and the matters described in thevarious embodiments may be applied independently or in combination oftwo or more.

Various embodiments of the present disclosure may be implemented inhardware, firmware, software, or a combination thereof. In the case ofimplementing the present disclosure by hardware, the present disclosurecan be implemented with application specific integrated circuits(ASICs), Digital signal processors (DSPs), digital signal processingdevices (DSPDs), programmable logic devices (PLDs), field programmablegate arrays (FPGAs), general processors, controllers, microcontrollers,microprocessors, etc.

In addition, the image decoding apparatus and the image encodingapparatus, to which the embodiments of the present disclosure areapplied, may be included in a multimedia broadcasting transmission andreception device, a mobile communication terminal, a home cinema videodevice, a digital cinema video device, a surveillance camera, a videochat device, a real time communication device such as videocommunication, a mobile streaming device, a storage medium, a camcorder,a video on demand (VoD) service providing device, an OTT video (over thetop video) device, an Internet streaming service providing device, athree-dimensional (3D) video device, a video telephony video device, amedical video device, and the like, and may be used to process videosignals or data signals. For example, the OTT video devices may includea game console, a blu-ray player, an Internet access TV, a home theatersystem, a smartphone, a tablet PC, a digital video recorder (DVR), orthe like.

FIG. 54 is a view showing a content streaming system, to which anembodiment of the present disclosure is applicable.

As shown in FIG. 54, the content streaming system, to which theembodiment of the present disclosure is applied, may largely include anencoding server, a streaming server, a web server, a media storage, auser device, and a multimedia input device.

The encoding server compresses content input from multimedia inputdevices such as a smartphone, a camera, a camcorder, etc. into digitaldata to generate a bitstream and transmits the bitstream to thestreaming server. As another example, when the multimedia input devicessuch as smartphones, cameras, camcorders, etc. directly generate abitstream, the encoding server may be omitted.

The bitstream may be generated by an image encoding method or an imageencoding apparatus, to which the embodiment of the present disclosure isapplied, and the streaming server may temporarily store the bitstream inthe process of transmitting or receiving the bitstream.

The streaming server transmits the multimedia data to the user devicebased on a user's request through the web server, and the web serverserves as a medium for informing the user of a service. When the userrequests a desired service from the web server, the web server maydeliver it to a streaming server, and the streaming server may transmitmultimedia data to the user. In this case, the content streaming systemmay include a separate control server. In this case, the control serverserves to control a command/response between devices in the contentstreaming system.

The streaming server may receive content from a media storage and/or anencoding server. For example, when the content are received from theencoding server, the content may be received in real time. In this case,in order to provide a smooth streaming service, the streaming server maystore the bitstream for a predetermined time.

Examples of the user device may include a mobile phone, a smartphone, alaptop computer, a digital broadcasting terminal, a personal digitalassistant (PDA), a portable multimedia player (PMP), navigation, a slatePC, tablet PCs, ultrabooks, wearable devices (e.g., smartwatches, smartglasses, head mounted displays), digital TVs, desktops computer, digitalsignage, and the like.

Each server in the content streaming system may be operated as adistributed server, in which case data received from each server may bedistributed.

The scope of the disclosure includes software or machine-executablecommands (e.g., an operating system, an application, firmware, aprogram, etc.) for enabling operations according to the methods ofvarious embodiments to be executed on an apparatus or a computer, anon-transitory computer-readable medium having such software or commandsstored thereon and executable on the apparatus or the computer.

INDUSTRIAL APPLICABILITY

The embodiments of the present disclosure may be used to encode ordecode an image.

1-15. (canceled)
 16. An image decoding method performed by an imagedecoding apparatus, the method comprising: deriving quantizationparameter of a current block based on a color space transform for aresidual sample of the current block; deriving transformationcoefficient of the current block based on the quantization parameter;deriving the residual sample of the current block based on thetransformation coefficient; and modifying a value of the residual samplebased on the color space transform being applied; wherein thequantization parameter is derived by clipping operation based on a valueof transformation skip flag of the current block; wherein the clippingoperation comprises adjusting a value of the quantization parameter tobe in a range from a predetermined lower value to a predetermined uppervalue, inclusive, wherein the predetermined upper value is derived basedon quantization parameter range offset and a predetermined constantvalue.
 17. The method of claim 16, wherein the predetermined constantvalue is
 63. 18. The method of claim 16, wherein the quantizationparameter is derived based on initial quantization parameter derivedbased on a color component of the current block.
 19. The method of claim16, wherein the quantization parameter is derived based on quantizationparameter offset derived based on a color component of the currentblock.
 20. The method of claim 19, wherein a size of the quantizationparameter offset is derived as 5 based on the color component of thecurrent block being a luma component and the color space transform beingapplied.
 21. The method of claim 19, wherein a size of the quantizationparameter offset is derived as 3 based on the color component of thecurrent block being a chroma Cr component and the color space transformbeing applied.
 22. The method of claim 19, wherein subtraction resultbased on a size of a first value of the quantization parameter offsetand a size of a second value of the quantization parameter offset is 2,wherein based on the color space transform being applied, the firstvalue is derived based on the quantization parameter offset of the colorcomponent of the current block being a chroma Cr component, and thesecond value is derived based on the quantization parameter offset ofthe color component of the current block being a chroma Cb component.23. The method of claim 16, wherein the predetermined lower value isderived based on the transformation skip flag of the current block. 24.The method of claim 16, wherein based on the transformation skip flagindicating not applying the transformation for the current block, thepredetermined lower value is derived as minimum allowed quantizationparameter for transform skip mode.
 25. The method of claim 16, whereinbased on the information on applying transformation indicating thatapplying the transformation for the current block is determined by othersyntax elements, the predetermined lower value is derived as
 0. 26. Themethod of claim 16, wherein a value of the quantization parameter rangeoffset is derived based on a syntax element for a bit depth of thecurrent block.
 27. An image encoding method performed by an imageencoding apparatus, the method comprising: deriving a value of aresidual sample of a current block based on a color space transformbeing applied; deriving transformation coefficient of the current blockbased on the residual sample of the current block; determiningquantization parameter of the current block based on the color spacetransform being applied; and encoding the transformation coefficientbased on the quantization parameter; wherein the quantization parameteris derived by clipping operation based on a value of transformation skipflag of the current block; wherein the clipping operation comprisesadjusting a value of the quantization parameter to be in a range from apredetermined lower value to a predetermined upper value, inclusive,wherein the predetermined upper value is derived based on quantizationparameter range offset and a predetermined constant value.
 28. Themethod of claim 27, wherein the predetermined constant value is
 63. 29.A method of transmitting a bitstream generated by the image encodingmethod of claim
 27. 30. A non-transitory computer-readable mediumstoring a bitstream generated by the image encoding method of claim 27.